python - What's the difference between Model.query and session.query(Model) in SQLAlchemy?

Question

I'm a beginner in SQLAlchemy and found query can be done in 2 method:

Approach 1:

DBSession = scoped_session(sessionmaker())
class _Base(object):
    query = DBSession.query_property()

Base = declarative_base(cls=_Base)

class SomeModel(Base):
    key   = Column(Unicode, primary_key=True)
    value = Column(Unicode)

# When querying
result = SomeModel.query.filter(...)

Approach 2

DBSession = scoped_session(sessionmaker())
Base = declarative_base()

class SomeModel(Base):
    key   = Column(Unicode, primary_key=True)
    value = Column(Unicode)

# When querying
session = DBSession()
result = session.query(SomeModel).filter(...)

Is there any difference between them?

Answer 1

In the code above, there is no difference. This is because, in line 3 of the first example:

• the query property is explicitly bound to DBSession
• there is no custom Query object passed to query_property

As @petr-viktorin points out in the answer here, there must be a session available before you define your model in the first example, which might be problematic depending on the structure of your application.

If, however, you need a custom query that adds additional query parameters automatically to all queries, then only the first example will allow that. A custom query class that inherits from sqlalchemy.orm.query.Query can be passed as an argument to query_property. This question shows an example of that pattern.

Even if a model object has a custom query property defined on it, that property is not used when querying with session.query, as in the last line in the second example. This means something like the first example the only option if you need a custom query class.