在 SQLAlchemy Flask 中创建对象时出现 NoForeignKeysError

Question

I am a beginner with SQLAlchemy and I just did my first modules.py file for a Flask application. However, in the main app, when I try to create two objects of type user :

from models import user_presence,User,Activity_Presence
db.create_all()
u1 = User()
u2 = User()

I get the error that: sqlalchemy.exc.NoForeignKeysError: Can't find any foreign key relationships between 'activity_presence' and 'User_Presence'. I tried following the official tutorials, but I don't understand why there is an issue with the foreign key relationship. I also tried adding more fields, adding objects to the relationship, but I just can't figure out what the problem is. If you have any idea I would be very thankful. Sorry if the question is too much of a beginner one.

from api import db

user_presence = db.Table('User_Presence',
                db.Column('user_id', db.Integer, db.ForeignKey('user.id'), primary_key=True),
                db.Column('presence_id', db.Integer, db.ForeignKey('Activity_Presence.id'), primary_key=True)
                )

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    presence_activ= db.relationship('Activity_Presence', secondary=user_presence, lazy='subquery',
                           backref=db.backref('users', lazy=True))

class Activity_Presence(db.Model):
    id = db.Column(db.Integer, primary_key=True)

Answer 1

The error lies in the reference to the wrong table name.
The usual naming of the classes that represent the model is done in camelcase. This name is then converted to snakecase to provide the table name. If you should use an underscore in your class name, it will be retained and another one will be added for any subsequent capital letters.

As an an example:

class ActivityPresence(db.Model):
    __tablename__ = 'activity_presence'

class Activity_Presence(db.Model):
    __tablename__ = 'activity__presence'

So the working code is as follows.

user_presence = db.Table('user_presence',
    db.Column('user_id', db.Integer, db.ForeignKey('user.id'), primary_key=True),
    db.Column('presence_id', db.Integer, db.ForeignKey('activity_presence.id'), primary_key=True)
)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    presence_activ= db.relationship('ActivityPresence',
        secondary=user_presence,
        lazy='subquery',
        backref=db.backref('users', lazy=True))

class ActivityPresence(db.Model):
    id = db.Column(db.Integer, primary_key=True)