Flask/SQLAlchemy - Difference between association model and association table for many-to-many relationship?

Question

I started learning this stuff from the Flask Mega Tutorial. When he gets into Many-to-Many relationships, he creates an association table like this:

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

As I was searching for ways to add some metadata regarding a specific association between models, I found that you can store this kinda thing in the association table.. However the example of this I've found seems to make the association table an actual model.

class DepartmentEmployeeLink(Base):
    __tablename__ = 'department_employee_link'
    department_id = Column(Integer, ForeignKey('department.id'), primary_key=True)
    employee_id = Column(Integer, ForeignKey('employee.id'), primary_key=True)
    extra_data = Column(String(256))
    department = relationship(Department, backref=backref("employee_assoc"))
    employee = relationship(Employee, backref=backref("department_assoc"))

What is the difference between these two methods? Is the model method required to store metadata in the association table or can the same thing be accomplished with the top method?

Thanks!

Solution

My apologies, I finally stumbled across the answer in the SQLAlchemy docs...

https://docs.sqlalchemy.org/en/latest/orm/basic_relationships.html#many-to-many

...where they explicitly define the difference:

Many to Many adds an association table between two classes.

association_table = Table('association', Base.metadata,
    Column('left_id', Integer, ForeignKey('left.id')),
    Column('right_id', Integer, ForeignKey('right.id'))
)

The association object pattern is a variant on many-to-many: it’s used when your association table contains additional columns beyond those which are foreign keys to the left and right tables. Instead of using the secondary argument, you map a new class directly to the association table.

class Association(Base):
    __tablename__ = 'association'
    left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
    right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)

    extra_data = Column(String(50))

    left = relationship('Left', backref=backref('right_association'))
    right = relationship('Right', backref=backref('left_association'))

Where "Right" and "Left" are tables, defined normally:

class Left(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key = True)
    ...

class Right(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key = True)
    ...

So it's basically creating an association object to reference this extra information if you need to store anything in the association, otherwise it's not necessary to use the ORM layer and you can just create an association table.

Comments

[Chockomonkey]

@oski86 you're welcome! Sorting out the difference between the two was a big light-bulb-moment for me. It's all in the application of the extra_data column in the object pattern! If you don't need extra data tied to a specific pair of models, then you don't need the association object--a simpler association table will suffice!

[New Guy]

@Chockomonkey Was searching for this. Please clear this for me. Would this be like a one-to-many relation now between 'Association' and the other models? In that case we'll have to define the relationship in those models as well? Would be of great help.

[Chockomonkey]

@NewGuy Good question, thanks for asking! You found where my answer was lackluster, and I've since updated it with the fleshed-out relationship fields and added some table definitions too for clarity. Hope this helps!

[user2682863]

updated doc link: docs.sqlalchemy.org/en/latest/orm/…

[user2682863]

for many-to-many it should be back_populates instead of backref

[Chockomonkey]

@user2682863 I looked into this today. The documentation suggests that backref is legitimately used in this case, and in fact ends up calling back_populates itself. It is a shorthand in that when backref is used in the definition of the relationship in the Association object, it does not require the explicit definition of a relationship in the Left and Right models. One could argue that explicit is better, but indeed either will do.