slqlachemy joined load by variable declaration

Question

Suppose I got two models. Account and Question.

class Account(DeclarativeBase):
    __tablename__ = 'accounts'

    id = Column(Integer, primary_key=True)
    user_name = Column(Unicode(255), unique=True, nullable=False)

and my Question model be like:

class Question(DeclarativeBase):
    __tablename__ = 'questions'

    id = Column(Integer, primary_key=True)
    content = Column(Unicode(2500), nullable=False)
    account_id = Column(Integer, ForeignKey(
    'accounts.id', onupdate='CASCADE', ondelete='CASCADE'), nullable=False)
    account = relationship('Account', backref=backref('questions'))

I got a method that returns a question in json format from the provided question ID. when the method is like this below, it only returns the id the content and the account_id of the question.

    @expose('json')
    def question(self, question_id):
        return dict(questions=DBSession.query(Question).filter(Question.id == question_id).one())

but I need the user_name of Account to be included in the json response too. something weird (at least to me) is that I have to explicitly tell the method that the query result contains a relation to an Account and this way the account info will be included in the json response: I mean doing something like this

    @expose('json')
    def question(self, question_id):
        result = DBSession.query(Question).filter(Question.id == question_id).one()
        weird_variable = result.account.user_name
        return dict(question=result)

why do I have to do such thing? what is the reason behind this?

Ilja Everilä · Accepted Answer · 2016-09-23T22:00:10.997

From Relationship Loading Techniques:

By default, all inter-object relationships are lazy loading.

In other words in its default configuration the relationship account does not actually load the account data when you fetch a Question, but when you access the account attribute of a Question instance. This behaviour can be controlled:

from sqlalchemy.orm import joinedload

DBSession.query(Question).\
    filter(Question.id == question_id).\
    options(joinedload('account')).\
    one()

Adding the joinedload option instructs the query to load the relationship at the same time with the parent, using a join. Other eager loading techniques are also available, their possible use cases and tradeoffs discussed under "What Kind of Loading to Use?"

slqlachemy joined load by variable declaration

1 Answers1