The lazy me you mean :)
I saw that option in the documents and my comment was not regarding the
complexity of the solution.
I just thought that a 'load_only' method is better understood if it loads
only the specified attributes.
With an abstraction layer, the user often "sees" things differently than
the way they really are.
For example, when I load the Foo object and I get the bars list, I don't
want to care about 'bars' being a relationship (I know it is but I already
handled that when I defined my model). For me, it's an attribute of type
list...
Of course some operations are relationship-specific but those that are not
should not (IMO) force the user to consider them differently.
Since I have my own wrapper around session.query, I added the following
lines to it:
query = session.query(model)
if properties is not None:
props = [p for p in properties if p in model.__table__.c]
query = query.options(sa.orm.load_only(*props))
noload = set([x.key for x in model.__mapper__.relationships]) - set(
props)
for x in noload:
query = query.options(sa.orm.lazyload(x))
...
That currently does what I want...
On Thursday, December 3, 2015 at 5:23:55 PM UTC+2, Michael Bayer wrote:
>
>
>
> On 12/03/2015 10:08 AM, Mike Bayer wrote:
> >
> >
> > On 12/03/2015 06:16 AM, Ofir Herzas wrote:
> >> load_only as stated
> >> in http://docs.sqlalchemy.org/en/latest/orm/loading_columns.html does
> >> the following:
> >> "An arbitrary set of columns can be selected as “load only” columns,
> >> which will be loaded *while deferring all other columns* on a given
> >> entity..."
> >
> >
> > Hi Ofir -
> >
> > As it says, "on a given entity", an entity refers to a class-oriented
> > object. A relationship refers to a different set of entities so these
> > must be referred to independently.
> >
> > The "load_only" function in that code example is hyperlinked, which is
> > intended to encourage the reader to click and read through the more
> > detailed documentation for this function which is at:
> >
> >
> http://docs.sqlalchemy.org/en/rel_1_0/orm/loading_columns.html?highlight=load_only#sqlalchemy.orm.load_only
>
> >
> >
> >
> >>
> >> However, joined relationships are not affected by this, meaning that
> the
> >> produced sql still joins all 'joined' relationships
> >>
> >> Consider the following example:
> >>
> >> |
> >> importsqlalchemy assa
> >> fromsqlalchemy.ext.declarative importdeclarative_base
> >>
> >> Base=declarative_base()
> >>
> >> classFoo(Base):
> >> __tablename__ ='foo'
> >> id =sa.Column(sa.Integer,primary_key=True)
> >> bars =sa.orm.relationship("Bar",lazy="joined")
> >>
> >> classBar(Base):
> >> __tablename__ ='bar'
> >> id =sa.Column(sa.Integer,primary_key=True)
> >> foo_id =sa.Column(sa.Integer,sa.ForeignKey('foo.id'))
> >> baz =sa.Column(sa.Integer)
> >>
> >> e =sa.create_engine("sqlite://",echo=True)
> >> session =sa.orm.Session(e)
> >> session.query(Foo).options(sa.orm.load_only('id')).all()
> >> |
> >>
> >> The query produced is the following:
> >> |
> >> SELECT foo.id AS foo_id,bar_1.id AS bar_1_id,bar_1.foo_id AS
> >> bar_1_foo_id,bar_1.baz AS bar_1_baz
> >> FROM foo LEFT OUTER JOIN bar AS bar_1 ON foo.id =bar_1.foo_id
> >> |
> >>
> >> I understand that I can use the lazyload option to prevent the join but
> >> I thought it would be nice if the load_only handled it
> out-of-the-box...
> >
> > there's "out of the box" and there's "assuming a specific and arbitrary
> > intent" - you have lazy="joined" in your mapping so if you don't want
> > that to occur you need to tell the query to turn it off, load_only('a',
> > 'b').lazyload('bars').
>
> another thought, you'll probably like this:
>
> load_only('a', 'b').lazyload('*')
>
>
> that should be easier
>
>
>
>
> >
> >
> >
> >
> >
> >>
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