How do you limit/specify the columns that are loaded via a relationship()?

[Seth]

Is there any way to get SQLAlchemy to only load specific columns through a relationship()?

For example, with this scenario:

class Parent(Base):

    __tablename__ = 'parent'

    id = Column(Integer, primary_key=True)

    children = relationship("Child", lazy='joined')

class Child(Base):

    __tablename__ = 'child'

    id = Column(Integer, primary_key=True)

    parent_id = Column(Integer, ForeignKey('parent.id'))

    cheap_column = Column(Unicode(10))

    expensive_column = Column(LargeBinary)

I'd like Child to get lazily joined when Parent is loaded, but only with Child's "cheap_column" and not it's "expensive_column".

Thanks,
Seth

[Michael Bayer]

session.query(Parent).options(defaultload(“children”).load_only(“cheap_column"))

or really if you want to cut out “expensive_column”

session.query(Parent).options(defaultload(“children”).defer(“expensive_column”))

http://docs.sqlalchemy.org/en/rel_0_9/orm/mapper_config.html#deferred

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[Seth]

Thanks Mike,

But...is there no way to set this behavior directly on the "relationship()"? The whole point there is to be "lazy" ;)

Seth

[Michael Bayer]

there are deferred() columns that you can set on a mapper but there’s not a mechanism right now to set deferred target attributes when the object is loaded only from a specific relationship without the specific call within the query().    So unless you maybe ran those options into every Query unconditionally, or something.  I’d look to see how big a deal it is to just be explicit on this one.

[Seth]

Ok, thanks so much for your help.

[Jonathan Vanasco]

Michael-

Quick question for clarity...

I have a table with a few deferred columns.  If I want to "eagerly" load them during a query, I should pass in the "undefer" option right ?

[Michael Bayer]

yeah there’s some bad history with the API here, in that you can’t easily pass them all at once, but right now it’s  q.options(undefer(‘*’)) for everything or q.options(undefer(‘x’), undefer(‘y’), …).  or undefer_group() if you’ve applied a group to them.

[Jonathan Vanasco]

that's not too bad.  at least for me; i only defer a few HSTORE columns.