SQLalchemy以声明方式描述关联对象的正确方法是什么

Question

I'm looking for a way to describe an associative object the declarative way. Beyond storing the foreign keys in the association table, I need to store information like the creation date of the association.

Today, my model looks like that :

# Define the User class
class User(Base):
    __tablename__ = 'users'

    # Define User fields
    id = schema.Column(types.Integer(unsigned=True),
        schema.Sequence('users_seq_id', optional=True), primary_key=True)
    password = schema.Column(types.Unicode(64), nullable=False)

# Define the UserSubset class
class UserSubset(Base):
    __tablename__ = 'subsets'

    # Define UserSubset fields
    id = schema.Column(types.Integer(unsigned=True),
        schema.Sequence('subsets_seq_id', optional=True), primary_key=True)
    some_short_description = schema.Column(types.Unicode(50), nullable=False)

# Define the subset memberships table
subset_memberships = schema.Table('group_memberships', Base.metadata,
    schema.Column('user_id', types.Integer(unsigned=True), ForeignKey('users.id')),
    schema.Column('subset_id', types.Integer(unsigned=True), ForeignKey('subsets.id')),
    schema.Column('created', types.DateTime(), default=now, nullable=False),
)

Can I connect everything in an associative object ? Or should I change stop using the declarative way ?

Answer 1

What you are using at the moment is just a Many-to-Many-relation. How to work with association objects is described in the docs .

There is also an extension called associationproxy which simplifies the relation.

Answer 2

As you can see in the manual , configuring a one to many relation is really simple:

class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    addresses = relation("Address", backref="user")

class Address(Base):
    __tablename__ = 'addresses'

    id = Column(Integer, primary_key=True)
    email = Column(String(50))
    user_id = Column(Integer, ForeignKey('users.id'))

Many to many relations isn't much harder:

There's nothing special about many-to-many with declarative. The secondary argument to relation() still requires a Table object, not a declarative class. The Table should share the same MetaData object used by the declarative base:

  keywords = Table('keywords', Base.metadata, Column('author_id', Integer, ForeignKey('authors.id')), Column('keyword_id', Integer, ForeignKey('keywords.id')) ) class Author(Base): __tablename__ = 'authors' id = Column(Integer, primary_key=True) keywords = relation("Keyword", secondary=keywords) 

You should generally not map a class and also specify its table in a many-to-many relation, since the ORM may issue duplicate INSERT and DELETE statements.

Anyway, what you seem to be doing might be better served with inheritance . Of course, there can be complex table relations that will be a pathological case for the declarative way, but this doesn't seem to be one of them.

_{One more thing, code comments should state what the following code does ans why, not how it does it. Having a # Define the User class comment is almost like having a line of code saying a = 1 # assing value 1 to variable "a" .}