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

Question

I started learning this material from the Flask Mega Tutorial. When it falls into a many-to-many relationship, it creates an association table as follows:

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

Since I was looking for ways to add some metadata regarding a specific association between models, I found that you can store this similar thing in the association table. However, the example of this that I found seems to create an actual model association table.

 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 be done using the top method?

Thanks!

Answer 1

My apologies, I finally stumbled upon an answer in SQLAlchemy docs ...

http://docs.sqlalchemy.org/en/rel_1_0/orm/basic_relationships.html#many-to-many

... where they clearly define the difference:

Many to many adds a table of relationships between the 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 template is a many-to-many option: it is used when your association table contains additional columns, in addition to those that are foreign keys for the left and right tables. Instead of using a secondary argument, you map the 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 usually defined tables:

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

Thus, this is basically creating an association object to reference this additional information if you need to save something in the association, otherwise there is no need to use the ORM layer, and you can just create an association table.