2018-01-23T20:01:33Z

The Flask Mega-Tutorial Part VIII: Followers

This is the eighth installment of the Flask Mega-Tutorial series, in which I'm going to tell you how to implement a "followers" feature similar to that of Twitter and other social networks.

For your reference, below is a list of the articles in this series.

Note 1: If you are looking for the legacy version of this tutorial, it's here.

Note 2: If you would like to support my work on this blog, or just don't have patience to wait for weekly articles, I am offering the complete version of this tutorial packaged as an ebook or a set of videos. For more information, visit courses.miguelgrinberg.com.

In this chapter I am going to work on the application's database some more. I want users of the application to be able to easily choose which other users they want to follow. So I'm going to be expanding the database so that it can keep track of who is following whom, which is harder than you may think.

The GitHub links for this chapter are: Browse, Zip, Diff.

Database Relationships Revisited

I said above that I want to maintain a list of "followed" and "follower" users for each user. Unfortunately, a relational database does not have a list type that I can use for these lists, all there is are tables with records and relationships between these records.

The database has a table that represents users, so what's left is to come up with the proper relationship type that can model the follower/followed link. This is a good time to review the basic database relationship types:

One-to-Many

I have already used a one-to-many relationship in Chapter 4. Here is the diagram for this relationship:

One-to-many Relationship

The two entities linked by this relationship are users and posts. I say that a user has many posts, and a post has one user (or author). The relationship is represented in the database with the use of a foreign key on the "many" side. In the relationship above, the foreign key is the user_id field added to the posts table. This field links each post to the record of its author in the user table.

It is pretty clear that the user_id field provides direct access to the author of a given post, but what about the reverse direction? For the relationship to be useful I should be able to get the list of posts written by a given user. The user_id field in the posts table is also sufficient to answer this question, as databases have indexes that allow for efficient queries such us "retrieve all posts that have a user_id of X".

Many-to-Many

A many-to-many relationship is a bit more complex. As an example, consider a database that has students and teachers. I can say that a student has many teachers, and a teacher has many students. It's like two overlapped one-to-many relationships from both ends.

For a relationship of this type I should be able to query the database and obtain the list of teachers that teach a given student, and the list of students in a teacher's class. This is actually non-trivial to represent in a relational database, as it cannot be done by adding foreign keys to the existing tables.

The representation of a many-to-many relationship requires the use of an auxiliary table called an association table. Here is how the database would look for the students and teachers example:

many-to-many

While it may not seem obvious at first, the association table with its two foreign keys is able to efficiently answer all the queries about the relationship.

Many-to-One and One-to-One

A many-to-one is similar to a one-to-many relationship. The difference is that this relationship is looked at from the "many" side.

A one-to-one relationship is a special case of a one-to-many. The representation is similar, but a constraint is added to the database to prevent the "many" side to have more than one link. While there are cases in which this type of relationship is useful, it isn't as common as the other types.

Representing Followers

Looking at the summary of all the relationship types, it is easy to determine that the proper data model to track followers is the many-to-many relationship, because a user follows many users, and a user has many followers. But there is a twist. In the students and teachers example I had two entities that were related through the many-to-many relationship. But in the case of followers, I have users following other users, so there is just users. So what is the second entity of the many-to-many relationship?

The second entity of the relationship is also the users. A relationship in which instances of a class are linked to other instances of the same class is called a self-referential relationship, and that is exactly what I have here.

Here is a diagram of the self-referential many-to-many relationship that keeps track of followers:

many-to-many

The followers table is the association table of the relationship. The foreign keys in this table are both pointing at entries in the user table, since it is linking users to users. Each record in this table represents one link between a follower user and a followed user. Like the students and teachers example, a setup like this one allows the database to answer all the questions about followed and follower users that I will ever need. Pretty neat.

Database Model Representation

Let's add followers to the database first. Here is the followers association table:

app/models.py: Followers association table

followers = db.Table('followers',
    db.Column('follower_id', db.Integer, db.ForeignKey('user.id')),
    db.Column('followed_id', db.Integer, db.ForeignKey('user.id'))
)

This is a direct translation of the association table from my diagram above. Note that I am not declaring this table as a model, like I did for the users and posts tables. Since this is an auxiliary table that has no data other than the foreign keys, I created it without an associated model class.

Now I can declare the many-to-many relationship in the users table:

app/models.py: Many-to-many followers relationship

class User(UserMixin, db.Model):
    # ...
    followed = db.relationship(
        'User', secondary=followers,
        primaryjoin=(followers.c.follower_id == id),
        secondaryjoin=(followers.c.followed_id == id),
        backref=db.backref('followers', lazy='dynamic'), lazy='dynamic')

The setup of the relationship is non-trivial. Like I did for the posts one-to-many relationship, I'm using the db.relationship function to define the relationship in the model class. This relationship links User instances to other User instances, so as a convention let's say that for a pair of users linked by this relationship, the left side user is following the right side user. I'm defining the relationship as seen from the left side user with the name followed, because when I query this relationship from the left side I will get the list of followed users (i.e those on the right side). Let's examine all the arguments to the db.relationship() call one by one:

  • 'User' is the right side entity of the relationship (the left side entity is the parent class). Since this is a self-referential relationship, I have to use the same class on both sides.
  • secondary configures the association table that is used for this relationship, which I defined right above this class.
  • primaryjoin indicates the condition that links the left side entity (the follower user) with the association table. The join condition for the left side of the relationship is the user ID matching the follower_id field of the association table. The followers.c.follower_id expression references the follower_id column of the association table.
  • secondaryjoin indicates the condition that links the right side entity (the followed user) with the association table. This condition is similar to the one for primaryjoin, with the only difference that now I'm using followed_id, which is the other foreign key in the association table.
  • backref defines how this relationship will be accessed from the right side entity. From the left side, the relationship is named followed, so from the right side I am going to use the name followers to represent all the left side users that are linked to the target user in the right side. The additional lazy argument indicates the execution mode for this query. A mode of dynamic sets up the query to not run until specifically requested, which is also how I set up the posts one-to-many relationship.
  • lazy is similar to the parameter of the same name in the backref, but this one applies to the left side query instead of the right side.

Don't worry if this is hard to understand. I will show you how to work with these queries in a moment, and then everything will become clearer.

The changes to the database need to be recorded in a new database migration:

(venv) $ flask db migrate -m "followers"
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.autogenerate.compare] Detected added table 'followers'
  Generating /home/miguel/microblog/migrations/versions/ae346256b650_followers.py ... done

(venv) $ flask db upgrade
INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 37f06a334dbf -> ae346256b650, followers

Adding and Removing "follows"

Thanks to the SQLAlchemy ORM, a user following another user can be recorded in the database working with the followed relationship as if it was a list. For example, if I had two users stored in user1 and user2 variables, I can make the first follow the second with this simple statement:

user1.followed.append(user2)

To stop following the user, then I could do:

user1.followed.remove(user2)

Even though adding and removing followers is fairly easy, I want to promote reusability in my code, so I'm not going to sprinkle "appends" and "removes" through the code. Instead, I'm going to implement the "follow" and "unfollow" functionality as methods in the User model. It is always best to move the application logic away from view functions and into models or other auxiliary classes or modules, because as you will see later in this chapter, that makes unit testing much easier.

Below are the changes in the user model to add and remove relationships:

app/models.py: Add and remove followers

class User(UserMixin, db.Model):
    #...

    def follow(self, user):
        if not self.is_following(user):
            self.followed.append(user)

    def unfollow(self, user):
        if self.is_following(user):
            self.followed.remove(user)

    def is_following(self, user):
        return self.followed.filter(
            followers.c.followed_id == user.id).count() > 0

The follow() and unfollow() methods use the append() and remove() methods of the relationship object as I have shown above, but before they touch the relationship they use the is_following() supporting method to make sure the requested action makes sense. For example, if I ask user1 to follow user2, but it turns out that this following relationship already exists in the database, I do not want to add a duplicate. The same logic can be applied to unfollowing.

The is_following() method issues a query on the followed relationship to check if a link between two users already exists. You have seen me use the filter_by() method of the SQLAlchemy query object before, for example to find a user given its username. The filter() method that I'm using here is similar, but lower level, as it can include arbitrary filtering conditions, unlike filter_by() which can only check for equality to a constant value. The condition that I'm using in is_following() looks for items in the association table that have the left side foreign key set to the self user, and the right side set to the user argument. The query is terminated with a count() method, which returns the number of results. The result of this query is going to be 0 or 1, so checking for the count being 1 or greater than 0 is actually equivalent. Other query terminators you have seen me use in the past are all() and first().

Obtaining the Posts from Followed Users

Support for followers in the database is almost complete, but I'm actually missing one important feature. In the index page of the application I'm going to show blog posts written by all the people that are followed by the logged in user, so I need to come up with a database query that returns these posts.

The most obvious solution is to run a query that returns the list of followed users, which as you already know, it would be user.followed.all(). Then for each of these returned users I can run a query to get the posts. Once I have all the posts I can merge them into a single list and sort them by date. Sounds good? Well, not really.

This approach has a couple of problems. What happens if a user is following a thousand people? I would need to execute a thousand database queries just to collect all the posts. And then I will need to merge and sort the thousand lists in memory. As a secondary problem, consider that the application's home page will eventually have pagination implemented, so it will not display all the available posts but just the first few, with a link to get more if desired. If I'm going to display posts sorted by their date, how can I know which posts are the most recent of all followed users combined, unless I get all the posts and sort them first? This is actually an awful solution that does not scale well.

There is really no way to avoid this merging and sorting of blog posts, but doing it in the application results in a very inefficient process. This kind of work is what relational databases excel at. The database has indexes that allow it to perform the queries and the sorting in a much more efficient way that I can possibly do from my side. So what I really want is to come up with a single database query that defines the information that I want to get, and then let the database figure out how to extract that information in the most efficient way.

Below you can see this query:

app/models.py: Followed posts query

class User(UserMixin, db.Model):
    #...
    def followed_posts(self):
        return Post.query.join(
            followers, (followers.c.followed_id == Post.user_id)).filter(
                followers.c.follower_id == self.id).order_by(
                    Post.timestamp.desc())

This is by far the most complex query I have used on this application. I'm going to try to decipher this query one piece at a time. If you look at the structure of this query, you are going to notice that there are three main sections designed by the join(), filter() and order_by() methods of the SQLAlchemy query object:

Post.query.join(...).filter(...).order_by(...)

Joins

To understand what a join operation does, let's look at an example. Let's assume that I have a User table with the following contents:

id username
1 john
2 susan
3 mary
4 david

To keep things simple I am not showing all the fields in the user model, just the ones that are important for this query.

Let's say that the followers association table says that user john is following users susan and david, user susan is following mary and user mary is following david. The data that represents the above is this:

follower_id followed_id
1 2
1 4
2 3
3 4

Finally, the posts table contains one post from each user:

id text user_id
1 post from susan 2
2 post from mary 3
3 post from david 4
4 post from john 1

This table also omits some fields that are not part of this discussion.

Here is the join() call that I defined for this query once again:

Post.query.join(followers, (followers.c.followed_id == Post.user_id))

I'm invoking the join operation on the posts table. The first argument is the followers association table, and the second argument is the join condition. What I'm saying with this call is that I want the database to create a temporary table that combines data from posts and followers tables. The data is going to be merged according to the condition that I passed as argument.

The condition that I used says that the followed_id field of the followers table must be equal to the user_id of the posts table. To perform this merge, the database will take each record from the posts table (the left side of the join) and append any records from the followers table (the right side of the join) that match the condition. If multiple records in followers match the condition, then the post entry will be repeated for each. If for a given post there is no match in followers, then that post record is not part of the join.

With the example data I defined above, the result of the join operation is:

id text user_id follower_id followed_id
1 post from susan 2 1 2
2 post from mary 3 2 3
3 post from david 4 1 4
3 post from david 4 3 4

Note how the user_id and followed_id columns are equal in all cases, as this was the join condition. The post from user john does not appear in the joined table because there are no entries in followers that have john as a followed user, or in other words, nobody is following john. And the post from david appears twice, because that user is followed by two different users.

It may not be immediately clear what do I gain by creating this join, but keep reading, as this is just one part of the bigger query.

Filters

The join operation gave me a list of all the posts that are followed by some user, which is a lot more data that I really want. I'm only interested in a subset of this list, the posts followed by a single user, so I need trim all the entries I don't need, which I can do with a filter() call.

Here is the filter portion of the query:

filter(followers.c.follower_id == self.id)

Since this query is in a method of class User, the self.id expression refers to the user ID of the user I'm interested in. The filter() call selects the items in the joined table that have the follower_id column set to this user, which in other words means that I'm keeping only the entries that have this user as a follower.

Let's say the user I'm interested in is john, which has its id field set to 1. Here is how the joined table looks after the filtering:

id text user_id follower_id followed_id
1 post from susan 2 1 2
3 post from david 4 1 4

And these are exactly the posts that I wanted!

Remember that the query was issued on the Post class, so even though I ended up with a temporary table that was created by the database as part of this query, the result will be the posts that are included in this temporary table, without the extra columns added by the join operation.

Sorting

The final step of the process is to sort the results. The part of the query that does that says:

order_by(Post.timestamp.desc())

Here I'm saying that I want the results sorted by the timestamp field of the post in descending order. With this ordering, the first result will be the most recent blog post.

Combining Own and Followed Posts

The query that I'm using in the followed_posts() function is extremely useful, but has one limitation. People expect to see their own posts included in their timeline of followed users, and the query as it is does not have that capability.

There are two possible ways to expand this query to include the user's own posts. The most straightforward way is to leave the query as it is, but make sure all users are following themselves. If you are your own follower, then the query as shown above will find your own posts along with those of all the people you follow. The disadvantage of this method is that it affects the stats regarding followers. All follower counts are going to be inflated by one, so they'll have to be adjusted before they are shown. The second way to do this is by create a second query that returns the user's own posts, and then use the "union" operator to combine the two queries into a single one.

After considering both options I decided to go with the second one. Below you can see the followed_posts() function after it has been expanded to include the user's posts through a union:

app/models.py: Followed posts query with user's own posts.

    def followed_posts(self):
        followed = Post.query.join(
            followers, (followers.c.followed_id == Post.user_id)).filter(
                followers.c.follower_id == self.id)
        own = Post.query.filter_by(user_id=self.id)
        return followed.union(own).order_by(Post.timestamp.desc())

Note how the followed and own queries are combined into one, before the sorting is applied.

Unit Testing the User Model

While I don't consider the followers implementation I have built a "complex" feature, I think it is also not trivial. My concern when I write non-trivial code, is to ensure that this code will continue to work in the future, as I make modifications on different parts of the application. The best way to ensure that code you have already written continues to work in the future is to create a suite of automated tests that you can re-run each time changes are made.

Python includes a very useful unittest package that makes it easy to write and execute unit tests. Let's write some unit tests for the existing methods in the User class in a tests.py module:

tests.py: User model unit tests.

from datetime import datetime, timedelta
import unittest
from app import app, db
from app.models import User, Post

class UserModelCase(unittest.TestCase):
    def setUp(self):
        app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite://'
        db.create_all()

    def tearDown(self):
        db.session.remove()
        db.drop_all()

    def test_password_hashing(self):
        u = User(username='susan')
        u.set_password('cat')
        self.assertFalse(u.check_password('dog'))
        self.assertTrue(u.check_password('cat'))

    def test_avatar(self):
        u = User(username='john', email='john@example.com')
        self.assertEqual(u.avatar(128), ('https://www.gravatar.com/avatar/'
                                         'd4c74594d841139328695756648b6bd6'
                                         '?d=identicon&s=128'))

    def test_follow(self):
        u1 = User(username='john', email='john@example.com')
        u2 = User(username='susan', email='susan@example.com')
        db.session.add(u1)
        db.session.add(u2)
        db.session.commit()
        self.assertEqual(u1.followed.all(), [])
        self.assertEqual(u1.followers.all(), [])

        u1.follow(u2)
        db.session.commit()
        self.assertTrue(u1.is_following(u2))
        self.assertEqual(u1.followed.count(), 1)
        self.assertEqual(u1.followed.first().username, 'susan')
        self.assertEqual(u2.followers.count(), 1)
        self.assertEqual(u2.followers.first().username, 'john')

        u1.unfollow(u2)
        db.session.commit()
        self.assertFalse(u1.is_following(u2))
        self.assertEqual(u1.followed.count(), 0)
        self.assertEqual(u2.followers.count(), 0)

    def test_follow_posts(self):
        # create four users
        u1 = User(username='john', email='john@example.com')
        u2 = User(username='susan', email='susan@example.com')
        u3 = User(username='mary', email='mary@example.com')
        u4 = User(username='david', email='david@example.com')
        db.session.add_all([u1, u2, u3, u4])

        # create four posts
        now = datetime.utcnow()
        p1 = Post(body="post from john", author=u1,
                  timestamp=now + timedelta(seconds=1))
        p2 = Post(body="post from susan", author=u2,
                  timestamp=now + timedelta(seconds=4))
        p3 = Post(body="post from mary", author=u3,
                  timestamp=now + timedelta(seconds=3))
        p4 = Post(body="post from david", author=u4,
                  timestamp=now + timedelta(seconds=2))
        db.session.add_all([p1, p2, p3, p4])
        db.session.commit()

        # setup the followers
        u1.follow(u2)  # john follows susan
        u1.follow(u4)  # john follows david
        u2.follow(u3)  # susan follows mary
        u3.follow(u4)  # mary follows david
        db.session.commit()

        # check the followed posts of each user
        f1 = u1.followed_posts().all()
        f2 = u2.followed_posts().all()
        f3 = u3.followed_posts().all()
        f4 = u4.followed_posts().all()
        self.assertEqual(f1, [p2, p4, p1])
        self.assertEqual(f2, [p2, p3])
        self.assertEqual(f3, [p3, p4])
        self.assertEqual(f4, [p4])

if __name__ == '__main__':
    unittest.main(verbosity=2)

I have added four tests that exercise the password hashing, user avatar and followers functionality in the user model. The setUp() and tearDown() methods are special methods that the unit testing framework executes before and after each test respectively. I have implemented a little hack in setUp(), to prevent the unit tests from using the regular database that I use for development. By changing the application configuration to sqlite:// I get SQLAlchemy to use an in-memory SQLite database during the tests. The db.create_all() call creates all the database tables. This is a quick way to create a database from scratch that is useful for testing. For development and production use I have already shown you how to create database tables through database migrations.

You can run the entire test suite with the following command:

(venv) $ python tests.py
test_avatar (__main__.UserModelCase) ... ok
test_follow (__main__.UserModelCase) ... ok
test_follow_posts (__main__.UserModelCase) ... ok
test_password_hashing (__main__.UserModelCase) ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.494s

OK

From now on, every time a change is made to the application, you can re-run the tests to make sure the features that are being tested have not been affected. Also, each time another feature is added to the application, a unit test should be written for it.

Integrating Followers with the Application

The support of followers in the database and models is now complete, but I don't have any of this functionality incorporated into the application, so I'm going to add that now.

Because the follow and unfollow actions introduce changes in the application, I'm going to implement them as POST requests, which are triggered from the web browser as a result of submitting a web form. It would be easier to implement these routes as GET requests, but then they could be exploited in CSRF attacks. Because GET requests are harder to protect against CSRF, they should only be used on actions that do not introduce state changes. Implementing these as a result of a form submission is better because then a CSRF token can be added to the form.

But how can a follow or unfollow action be triggered from a web form when the only thing the user needs to do is click on "Follow" or "Unfollow", without submitting any data? To make this work, the form is not going to have any data fields. The only elements in the form are going to be the CSRF token, which is implemented as a hidden field and added automatically by Flast-WTF, and a submit button, which is going to be what the user needs to click to trigger the action. Since the two actions are almost identical I'm going to use the same form for both. I'm going to call this form EmptyForm.

app/forms.py: Empty form for following and unfollowing.

class EmptyForm(FlaskForm):
    submit = SubmitField('Submit')

Let's add two new routes in the application to follow and unfollow a user:

app/routes.py: Follow and unfollow routes.

from app.forms import EmptyForm

# ...

@app.route('/follow/<username>', methods=['POST'])
@login_required
def follow(username):
    form = EmptyForm()
    if form.validate_on_submit():
        user = User.query.filter_by(username=username).first()
        if user is None:
            flash('User {} not found.'.format(username))
            return redirect(url_for('index'))
        if user == current_user:
            flash('You cannot follow yourself!')
            return redirect(url_for('user', username=username))
        current_user.follow(user)
        db.session.commit()
        flash('You are following {}!'.format(username))
        return redirect(url_for('user', username=username))
    else:
        return redirect(url_for('index'))


@app.route('/unfollow/<username>', methods=['POST'])
@login_required
def unfollow(username):
    form = EmptyForm()
    if form.validate_on_submit():
        user = User.query.filter_by(username=username).first()
        if user is None:
            flash('User {} not found.'.format(username))
            return redirect(url_for('index'))
        if user == current_user:
            flash('You cannot unfollow yourself!')
            return redirect(url_for('user', username=username))
        current_user.unfollow(user)
        db.session.commit()
        flash('You are not following {}.'.format(username))
        return redirect(url_for('user', username=username))
    else:
        return redirect(url_for('index'))

The form handling in these routes is simpler, because we only have to implement the submission part. Unlike other forms such as the login and edit profile forms, these two forms do not have their own pages, the forms will be rendered by the user() route and will appear in the user's profile page. The only reason why the validate_on_submit() call can fail is if the CSRF token is missing or invalid, so in that case I just redirect the application back to the home page.

If the form validation passes, I do some error checking before actually carrying out the follow or unfollow action. This is to prevent unexpected issues, and to try to provide a useful message to the user when a problem has occurred.

To render the follow or unfollow button, I need to instantiate an EmptyForm object and pass it to the user.html template. Because these two actions are mutually exclusive, I can pass a single instance of this generic form to the template:

app/routes.py: Follow and unfollow routes.

@app.route('/user/<username>')
@login_required
def user(username):
    # ...
    form = EmptyForm()
    return render_template('user.html', user=user, posts=posts, form=form)

I can now add the follow or unfollow forms in the profile page of each user:

app/templates/user.html: Follow and unfollow links in user profile page.

        ...
        <h1>User: {{ user.username }}</h1>
        {% if user.about_me %}<p>{{ user.about_me }}</p>{% endif %}
        {% if user.last_seen %}<p>Last seen on: {{ user.last_seen }}</p>{% endif %}
        <p>{{ user.followers.count() }} followers, {{ user.followed.count() }} following.</p>
        {% if user == current_user %}
        <p><a href="{{ url_for('edit_profile') }}">Edit your profile</a></p>
        {% elif not current_user.is_following(user) %}
        <p>
            <form action="{{ url_for('follow', username=user.username) }}" method="post">
                {{ form.hidden_tag() }}
                {{ form.submit(value='Follow') }}
            </form>
        </p>
        {% else %}
        <p>
            <form action="{{ url_for('unfollow', username=user.username) }}" method="post">
                {{ form.hidden_tag() }}
                {{ form.submit(value='Unfollow') }}
            </form>
        </p>
        {% endif %}
        ...

The changes to the user profile template add a line below the last seen timestamp that shows how many followers and followed users this user has. And the line that has the "Edit" link when you are viewing your own profile now can have one of three possible links:

  • If the user is viewing his or her own profile, the "Edit" link shows as before.
  • If the user is viewing a user that is not currently followed, the "Follow" form shows.
  • If the user is viewing a user that is currently followed, the "Unfollow" form shows.

To reuse the EmptyForm() instance for both the follow and unfollow forms, I pass a value argument when rendering the submit button. In a submit button, the value attribute defines the label, so with this trick I can change the text in the submit button depending on the action that I need to present to the user.

At this point you can run the application, create a few users and play with following and unfollowing users. The only thing you need to remember is to type the profile page URL of the user you want to follow or unfollow, since there is currently no way to see a list of users. For example, if you want to follow a user with the susan username, you will need to type http://localhost:5000/user/susan in the browser's address bar to access the profile page for that user. Make sure you check how the followed and follower user counts change as you issue follows or unfollows.

I should be showing the list of followed posts in the index page of the application, but I don't have all the pieces in place to do that yet, since users cannot write blog posts yet. So I'm going to delay this change until that functionality is in place.

274 comments

  • #251 Luisangel Marcia said 2020-05-31T22:26:48Z

    Hello mr. Grindberg. I'm having a good time with you're mega tutorial. I have a question, I was trying to deal with this error but honestly I've got everything that you have implemented. sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) near "UNION": syntax error [SQL: SELECT anon_1.post_id AS anon_1_post_id, anon_1.post_body AS anon_1_post_body, anon_1.post_timestamp AS anon_1_post_timestamp, anon_1.post_user_id AS anon_1_post_user_id FROM ((SELECT post.id AS post_id, post.body AS post_body, post.timestamp AS post_timestamp, post.user_id AS post_user_id FROM post JOIN followers ON followers.followed_id = post.user_id WHERE followers.follower_id = ? ORDER BY post.timestamp DESC) UNION SELECT post.id AS post_id, post.body AS post_body, post.timestamp AS post_timestamp, post.user_id AS post_user_id FROM post WHERE post.user_id = ?) AS anon_1 ORDER BY anon_1.post_timestamp DESC] [parameters: (1, 1)] (Background on this error at: http://sqlalche.me/e/e3q8) I get it that is a problem with an Union that has not been set from the point of view of SQLAlchemy, nonetheless it keeps me bothering cause I have everything identical as you're tutorial. Do you know what maybe the issue here? Greetings from Nicaragua.

  • #252 Miguel Grinberg said 2020-06-01T09:00:38Z

    @Rostislav: my guess would be that you are not initializing your database before the test runs. Have you called db.create_all() to generate all the tables?

  • #253 Miguel Grinberg said 2020-06-01T09:30:17Z

    @Luisangel: I do not get the same query as you. Here is the SQL that I get here:

    'SELECT anon_1.post_id AS anon_1_post_id, anon_1.post_body AS anon_1_post_body, anon_1.post_timestamp AS anon_1_post_timestamp, anon_1.post_user_id AS anon_1_post_user_id \nFROM (SELECT post.id AS post_id, post.body AS post_body, post.timestamp AS post_timestamp, post.user_id AS post_user_id \nFROM post JOIN followers ON followers.followed_id = post.user_id \nWHERE followers.follower_id = ? UNION SELECT post.id AS post_id, post.body AS post_body, post.timestamp AS post_timestamp, post.user_id AS post_user_id \nFROM post \nWHERE post.user_id = ?) AS anon_1 ORDER BY anon_1.post_timestamp DESC'

    Something in your query must be different.

  • #254 Rostislav said 2020-06-01T11:30:36Z

    I'm using this post as a framework for doing something similar, but with a slight difference.

    instead of followed_posts - I'm trying to show posts from those you don't follow.

    Taking your code as a base - I switch out .filter(followers.c.follower_id == self.id) to .filter(followers.c.follower_id != self.id) - but am not getting the result that I expect. Instead of getting the posts where I'm not following, I'm getting all posts.

    def unfollowed_posts(self): followed = Post.query.join( followers, (followers.c.followed_id == Post.user_id)).filter( followers.c.follower_id != self.id)

    return unfollowed

    I'm guessing that != is not a valid column operator here? But I've scoured the web and can't find the proper way to do it. Rather, the sqlalchemy docs show that such an operator is valid, in which case I'm not sure why it doesn't work.

    Any idea what's the best way to go about this?

    if I open up app.db in sqlite3, i'm able to get the result I expect using a left join and an 'IS NOT' query.

  • #255 Rostislav said 2020-06-01T11:33:23Z

    @Miguel - re my previous issue with the db issue - like some of the other people in the comments, my issue was I didn't catch the capital "U" in def setUp(self) - somehow my eyes weren't attuned to it. And it was deleting my tables after the initial tests.

    Won't make that mistake again.

  • #256 shashidhar said 2020-06-03T07:10:29Z

    Hi Migual,

    First of all, Thanks for this great tutorial. Sorry for discussing of the topics of tutorial. But I am stuck with these from weeks. Your help will allow me to proceed. Please help. I am using your tutorial to build a small ticket management application. I have slightly different requirements in schema. Below are the model classes and requirements. Please help me with some pointers to tutorials, sample code or documents which I can use to build the schema.

    My requirements: 1. User can have many tickets, and ticket can have many users. 2. A ticket has many comments. 3. Every comment has one user, and user has many comments.

    models.py ticketuser = db.Table('ticketuser', db.Column('user_id', db.Integer, db.ForeignKey('user.id'), primary_key=True), db.Column('ticket_id', db.Integer, db.ForeignKey('ticket.id', primary_key=True)) ) class Comment(db.Model): id = db.Column(db.Integer, primary_key=True) text = db.Column(db.Text) user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False) ticket_id = db.Column(db.Integer, db.ForeignKey('ticket.id')) class Role(db.Model): id = db.Column(db.Integer, primary_key=True) role_name = db.Column(db.String(10)) class Ticket(db.Model): id = db.Column(db.Integer, primary_key=True) goal = db.Column(db.String(512), unique=True, nullable=False) status = db.Column(db.String(32), nullable=False) comments = db.relationship('Comment', backref = 'ticket', lazy='dynamic') users = db.relationship('User', secondary=ticketuser, backref=db.backref('tickets', lazy='dynamic') ) class User(UserMixin, db.Model): id = db.Column(db.Integer, primary_key=True) username = db.Column(db.String(64), index=True, unique=True) email = db.Column(db.String(120), index=True, unique=True) comments = db.relationship('Comment', backref='commenter', lazy='dynamic'
  • #257 Miguel Grinberg said 2020-06-03T11:07:05Z

    @shashidhar: I can't really see anything wrong with your models. Is anything not working?

  • #258 Stan Choi said 2020-06-04T22:33:46Z

    Thank you Mr. Grinberg. Please clarify my misunderstanding if any. In the code snippet below, it shouldn't matter, but in the def test_follow method as part of the unit testing, can one use u2.followers.all()? Thank you.

    def test_follow(self): u1 = User(username='john', email='john@example.com') u2 = User(username='susan', email='susan@example.com') db.session.add(u1) db.session.add(u2) db.session.commit() self.assertEqual(u1.followed.all(), []) self.assertEqual(u1.followers.all(), [])
  • #259 Miguel Grinberg said 2020-06-05T10:02:52Z

    @Stan: Yes, you can. It doesn't add anything of value to the test because u1 and u2 are identical, but sure, you can use it as well.

  • #260 Ankit said 2020-06-11T09:11:42Z

    Hi Miguel Good job sir , As a beginner, reading this blog is useful in understanding flask and a lot More about databases. Keep up the good work.Thank you one again.

  • #261 Michael Gardner said 2020-06-21T19:15:01Z

    In the video tutorial, you don't make use of the EmptyForm Class to prevent CSRF attacks? Is EmptyForm no longer needed or should I use the text-based implementation?

  • #262 Miguel Grinberg said 2020-06-21T21:40:50Z

    @Michael: the EmptyForm implementation is more robust. I added this after the videos were recorded. In general any time there is a difference between the videos and the text, you should give preference to the text.

  • #263 David Adeliyi said 2020-07-16T12:57:54Z

    Hello Miguel,

    All my tests are running fine but when I try to follow a user after starting the server, I get the following error which comes from the is_following( ) function. It appears the user.id reference isn't returning an id

    File "/Users/sadeliyi/Documents/Python Projects/microblog_app/app/models.py", line 47, in is_following return len(list(self.followed.filter(followers.c.followed_id == user.id))) > 0 AttributeError: 'BaseQuery' object has no attribute 'id'

    This is how my function looks. Could there be something wrong with the model? Though I have tried to use the shell to fetch users by id using the get( ) method and that works fine.

    def is_following(self, user): return self.followed.filter(followers.c.followed_id == user.id).count() &gt; 0 #return len(list(self.followed.filter(followers.c.followed_id == user.id))) &gt; 0
  • #264 Miguel Grinberg said 2020-07-18T13:51:11Z

    @David: the user variable should be a user object, but in your case it is a query object. That makes me think that when you are calling the user.follow() or user.unfollow() methods you are not passing a user object as an argument.

  • #265 Alan Sanders said 2020-07-19T13:50:39Z

    When I ran the test code I got an AssertionError:

    Traceback (most recent call last): File "tests.py", line 81, in test_follow_posts self.assertEqual(f1, [p2, p4, p1]) AssertionError: Lists differ: [<Post post from john>, <Post post from susan>, <Post post from david>] != [<Post post from susan>, <Post post from david>, <Post post from john>]

    First differing element 0: <Post post from john> <Post post from susan>

    To fix it I changed: self.assertEqual(f1, [p2, p4, p1]) to self.assertEqual(f1, [p1, p2, p4])

    Not sure why the order matters since the lists are equal in terms of what they contain, but the test failed for me until I changed the order.

  • #266 Miguel Grinberg said 2020-07-19T15:30:22Z

    @Alan: Hard to know without seeing the code, but it appear you are just getting the users in a different order. You must have some minor difference in your code that causes this.

  • #267 Oluwaseyi Adeliyi said 2020-07-19T20:56:13Z

    @Miguel Thanks for the help. I realized in my routes.py, when I query the user, I did not include the first() method. This was the reason

  • #268 Mohammad Ramzi Asal said 2020-07-21T10:34:15Z

    Hi Miguel, why am i getting such an unexpected error. sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) table follow_ship_table has no column named follower_id [SQL: INSERT INTO follow_ship_table (follower_id, followed_id) VALUES (?, ?)] [parameters: (4, 1)]

  • #269 Miguel Grinberg said 2020-07-21T14:05:57Z

    @Mohammad: the error indicates that your follow_ship_table does not have a column named follower_id. Maybe you forgot to migrate your database after you added this column to your models?

  • #270 Jac said 2020-08-04T10:32:51Z

    Why do you overload the word "followers", using it for both the name of the auxiliary table and for the back reference for the right side to self relationship?

  • #271 Miguel Grinberg said 2020-08-04T10:45:38Z

    @Jac: No reason. The name suits both. Feel free to rename one of them if you find it confusing.

  • #272 hyun said 2020-08-07T15:44:30Z

    I am sorry to bombard this comment section. I want to delete/edit my previous comments but I can't seem to find them.. I followed your answer #229 and that fixed the problem. However, all the users that I had added before are gone? I ran "flask db upgrade" which applied the patch from "flask db migrate -m "followers"".

    Is this normal or did I do something wrong? Shouldn't it bring the old users & posts back?

  • #273 hyun said 2020-08-07T15:50:39Z

    Hi Miguel, how is this possible - "user.followers.count()" ? I can see that the "followers" is declared and defined in models.py but it is not a member variable of User. I checked your code in github and it is not a member.

  • #274 Miguel Grinberg said 2020-08-07T18:29:55Z

    @hyun: the migrations do not destroy any data that you have stored in your database. If your data disappeared it could be because you downgraded the database to a version prior to that data existing.

    The followers member is defined as a backref on the other side of the relationship.

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