Flask-SQLAlchemy Query Join relational tables

Question

I am building an app using Flask & SQLAlchemy. I basically have 3 tables: users, friendships and bestFriends:

A user can have many friends but only one best friend. So I want my model to be relational. 'One-to-many' for the relation between 'users' and 'friendships' & 'one-to-one' between 'users' and 'bestFriends'.

This is my Model:

from app import db
from sqlalchemy.orm import relationship, backref
from sqlalchemy import Table, Column, Integer, ForeignKey
from sqlalchemy.ext.declarative import declarative_base

class users(db.Model):

    __tablename__ = "Users"

    id = db.Column(db.Integer, primary_key=True)
    userName = db.Column(db.String, nullable=False)
    userEmail = db.Column(db.String, nullable=False)
    userPhone = db.Column(db.String, nullable=False)
    userPass = db.Column(db.String, nullable=False)

    friendsR = db.relationship('friendships', backref='friendships.friend_id', primaryjoin='users.id==friendships.user_id', lazy='joined')

    def __init__(self, userName, userEmail, userPhone, userPass):
        self.userName = userName
        self.userEmail = userEmail
        self.userPhone = userPhone
        self.userPass = userPass

    def __repr__(self):
        return '{}-{}-{}-{}'.format(self.id, self.userName, self.userEmail, self.userPhone)

class friendships(db.Model):

    __tablename__ = "Friendships"

    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, ForeignKey('Users.id'), nullable=False)
    friend_id = db.Column(db.Integer, ForeignKey('Users.id'), nullable=False)

    userR = relationship('users', foreign_keys='friendships.user_id')
    friendR = relationship('users', foreign_keys='friendships.friend_id')

    def __init__(self, user_id, friend_id):
        self.user_id = user_id
        self.friend_id = friend_id


    def __repr__(self):
        return '{}-{}-{}-{}'.format(self.user_id, self.friend_id)


class bestFriends(db.Model):

    __tablename__ = "BestFriends"

    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, ForeignKey('Users.id'), nullable=False)
    best_friend_id = db.Column(db.Integer, ForeignKey('Users.id'), nullable=False)

    user = relationship('users', foreign_keys='bestFriends.user_id')
    best_friend = relationship('users', foreign_keys='bestFriends.best_friend_id')


    def __init__(self, user_id, best_friend_id):

        self.user_id = user_id
        self.best_friend_id = best_friend_id


    def __repr__(self):
        return '{}-{}-{}-{}'.format(self.user_id, self.best_friend_id)

I need to be able to query the list of friends of the user logged in as well as the best friend of that user in case exists. I also need to PAGINATE the result:

Here is my app.py function for displaying the friends of a user:

@app.route('/friendList<int:page>', methods=['GET', 'POST'])
@app.route('/friends')
def friendList(page=1):

if not session.get('logged_in'):
        return render_template('login.html')
    else:
        userID = session['user_id']

        userList = users.query.join(friendships).add_columns(users.id, users.userName, users.userEmail, friendships.user_id, friendships.friend_id).filter(users.id == friendships.friend_id).filter(friendships.user_id == userID).paginate(page, 1, False)

        return render_template(
            'friends.html', userList=userList)

And this would be the Jinja side of the code:

{% extends "layout.html" %}
{% block body %}

<div id="pagination">
{% if userList.has_prev %}
        <a href="{{ url_for('friendList', page=userList.prev_num) }}">Back</a>
{% endif %} 

{% if userList.has_next %}
        <a href="{{ url_for('friendList', page=userList.next_num) }}">Next</a>
{% endif %}
</div>

<div style="clear:both;"></div>

<div id="innerContent">
{% if userList %}
    {% for friends in userList %}
                    <div class="contentUsers">
                        {{ friends.userName }}
                    </div>

                    <br><br><br><br>

    {% endfor %}{% else %}<div>No friends</div>
{% endif %} 

</div>
  {% endblock %}

And if I query like this:

userList = db.session.query(users,friendships).filter(users.id == friendships.friend_id).filter(friendships.user_id == userID).paginate(page, 1, False)

I get this error:

InvalidRequestError: Could not find a FROM clause to join from. Tried joining to <class 'models.friendships'>, but got: Can't determine join between 'Users' and 'Friendships'; tables have more than one foreign key constraint relationship between them. Please specify the 'onclause' of this join explicitly.

If I query like this:

userList = users.query.join(friendships, users.id==friendships.user_id).add_columns(users.id, users.userName, users.userEmail, friendships.user_id, friendships.friend_id).filter(users.id == friendships.friend_id).filter(friendships.user_id == userID).paginate(page, 1, False)

I get the following error:

TypeError: 'Pagination' object is not iterable

I still think this later query is right way to go but I think there is something wrong in my relationships/foreign keys between tables!!!

If on the Jinja side I add the .items to the loop:

{% if userList.items %}
{% for friends in userList.items %}

                <div class="contentUsers">
                    {{ friends.userName }}
                </div>

                <br><br><br><br>

{% endfor %}{% else %}<div>No friends</div>

{% endif %}

It doesn't loop at all and simply displays the else statement of "no friends"

Answer 1

The error message is telling you that SQLAlchemy can't determine how to join the two tables users and friendships, because there is more than one foreign key linking them. You need to explicitly define the join condition.

Try:

userList = users.query.join(friendships, users.id==friendships.user_id).add_columns(users.userId, users.name, users.email, friends.userId, friendId).filter(users.id == friendships.friend_id).filter(friendships.user_id == userID).paginate(page, 1, False)

Answer 2

Ok looks like after getting some sleep and viewing Matthewh's suggestion I almost found the final solution:

My model:

from app import db
from sqlalchemy.orm import relationship, backref
from sqlalchemy import Table, Column, Integer, ForeignKey
from sqlalchemy.ext.declarative import declarative_base

class users(db.Model):
    __tablename__ = "Users"

    id = db.Column(db.Integer, primary_key=True)
    userName = db.Column(db.String, nullable=False)
    userEmail = db.Column(db.String, nullable=False)
    userPhone = db.Column(db.String, nullable=False)
    userPass = db.Column(db.String, nullable=False)

    def __init__(self, userName, userEmail, userPhone, userPass):
        self.userName = userName
        self.userEmail = userEmail
        self.userPhone = userPhone
        self.userPass = userPass

    def __repr__(self):
        return '{}-{}-{}-{}'.format(self.id, self.userName, self.userEmail, self.userPhone)


 class friendships(db.Model):
    __tablename__ = "Friendships"

    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('Users.id'), nullable=False)
    friend_id = db.Column(db.Integer, db.ForeignKey('Users.id'), nullable=False)
    userR = db.relationship('users', foreign_keys='friendships.user_id')
    friendR = db.relationship('users', foreign_keys='friendships.friend_id')

    def __init__(self, user_id, friend_id):
        self.user_id = user_id
        self.friend_id = friend_id

    def __repr__(self):
        return '{}-{}-{}-{}'.format(self.user_id, self.friend_id)


class bestFriends(db.Model):
    __tablename__ = "BestFriends"

    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('Users.id'), nullable=False)
    best_friend_id = db.Column(db.Integer, db.ForeignKey('Users.id'), nullable=False)
    user = db.relationship('users', foreign_keys='bestFriends.user_id')
    best_friend = db.relationship('users', foreign_keys='bestFriends.best_friend_id')

    def __init__(self, user_id, best_friend_id):
        self.user_id = user_id
        self.best_friend_id = best_friend_id

    def __repr__(self):
        return '{}-{}-{}-{}'.format(self.user_id, self.best_friend_id)

My app.py function(shows friends of user logged in):

@app.route('/friendList<int:page>', methods=['GET', 'POST'])
@app.route('/friends')
  def friendList(page=1):
  if not session.get('logged_in'):
     return render_template('login.html')
  else:
     userID = session['user_id']
     userList = users.query.join(friendships, users.id==friendships.user_id).add_columns(users.id, users.userName, users.userEmail, friendships.id, friendships.user_id, friendships.friend_id).filter(friendships.friend_id == userID).paginate(page, 1, False)
     return render_template('friends.html', userList=userList)

And the Jinja side of 'friends.html':

{% extends "layout.html" %}

{% block body %}
    <div id="pagination">
        {% if userList.has_prev %}
            <a href="{{ url_for('friendList', page=userList.prev_num)}}">Back</a>
        {% endif %}

        {% if userList.has_next %}
            <a href="{{ url_for('friendList', page=userList.next_num)}}">Next</a>
        {% endif %}
    </div>

    <div style="clear:both;"></div>

    <div id="innerContent">
    {% if userList.items %}
        {% for friends in userList.items %}
            <div class="contentUsers">
                {{ friends.userName }}
            </div>
            <br><br><br><br>
        {% endfor %}
        {% else %}
            <div>No friends</div>
        {% endif %}
    </div>
{% endblock %}

This gives me an object(friends in userList.items) like this:

(2-Carlos-carlos@carlos.com-900102030, 2, u'Carlos', u'carlos@carlos.com', 2, 2, 1)

I was expecting this: |users.id|users.userName|users.userEmail|users.userPhone|friendships.id|friendships.user_id(the friends)| friendships.friend_id(the logged in user)|

So I have the following doubts/questions:

I am not completely understanding the structure of the object resulting for the query:

-repeated user id, name and email -what is the 'u' infront of second name and email

I am not completely understanding the relations model structure:

• why is the following NOT required in the 'users' class of the database model:

  #friendsR = db.relationship('friendships', backref='friendships.friend_id', primaryjoin='users.id==friendships.user_id', lazy='joined')
  

IS my database model correctly defined regarding standarized relationship model as posted in this answer or should I improove some how??