为什么SQLAlchemy中的ForeignKey和RelationshipProperty属性不会自动同步？(Why are not ForeignKey and RelationshipProperty attributes in SQLAlchemy automatically synchronized?)

 我试图弄清楚SQLAlchemy处理外键的方式。 
 
 考虑这些模型： 
 
class ModelOne(Base):
   __tablename__ = 'model_one'

   id = Column(Integer, primary_key=True)
   ...

class ModelTwo(Base):
   __tablename__ = 'model_two'

   id = Column(Integer, primary_key=True)
   one_id = Column(Integer, ForeignKey('model_one.id'))
   one = relationship('ModelOne')
 
 我假设如果我们有两个这样的模型，那么SQLAlchemy会发现ModelTwo.one_id和ModelTwo.one指的是同一个数据库列。 
 
 在创建ModelTwo记录时，我们可以使用ModelOne记录的数字ID或ModelOne实例来填充FK字段： 
 
one = DBSession.query(ModelOne)....
two_a = ModelTwo(one_id=one.id)
two_b = ModelTwo(one=one)
 
 这很有效。 
 
 但是这里有一个转折：在实例化ModelTwo实例后，根据我们初始化FK字段的方式，它的一个或另一个表示将保持未填充： 
 
two_a.one is None # evaluates to True
two_b.one_id is None # evaluates to True
 
 这是预期的行为吗？ 如果是这样，这必须是开发人员头上的一个巨大的红旗。 当我们从DB检索记录作为声明性模型类的实例时，将填充record.one和record.one_id，但是当我们创建新记录时，情况并非如此！ 

I am trying to figure out the way SQLAlchemy is handling foreign keys. 
 
Consider these models:
 
class ModelOne(Base):
   __tablename__ = 'model_one'

   id = Column(Integer, primary_key=True)
   ...

class ModelTwo(Base):
   __tablename__ = 'model_two'

   id = Column(Integer, primary_key=True)
   one_id = Column(Integer, ForeignKey('model_one.id'))
   one = relationship('ModelOne')
 
I assumed that if we have two models like this, then SQLAlchemy will figure out that ModelTwo.one_id and ModelTwo.one are referring to the same database column. 
 
When creating ModelTwo records, we can use either numerical ID of the ModelOne records, or ModelOne instances to fill the FK field:
 
one = DBSession.query(ModelOne)....
two_a = ModelTwo(one_id=one.id)
two_b = ModelTwo(one=one)
 
This works. 
 
But here's a twist: immediately after instantiating the ModelTwo instances, depending on how we initialize the FK field, one or the other representation of it will remain unpopulated:
 
two_a.one is None # evaluates to True
two_b.one_id is None # evaluates to True
 
Is this an expected behaviour? If so, this has to be a huge red flag in the head of a developer. When we retrieve a record from DB as an instance of a declarative model class, both record.one and record.one_id will be populated, but when we are creating a new record, that is not the case! 

原文：https://stackoverflow.com/questions/24685814