what it cant locate are foreign keys between the parent and child
tables, "a" and "c"....because there arent any. when you have a many-
to-many, the rules for figuring out the relationship change, and it
knows to do that by the presence of the "secondary" argument.
so if you can manufacture a "secondary" table you can do this:
secondary = a_b_table.join(b_c_table,
onclause=a_b_table.c.b_id==b_c_table.c.b_id)
mapper(
A, a_table,
properties={'cs': relation(C, secondary=secondary,
primaryjoin=a_table.c.id==secondary.c.a_b_a_id,
secondaryjoin=secondary.c.b_c_c_id==c_table.c.id,
viewonly=True,
)
}
)
im not totally sure the lazy clause is going to work but try it out.
this goes back to my general dislike of "viewonly" and how i cant
generally support it, becuase as the rules for relationships get more
strict and accurate, cases like these become harder to model.
On Mar 28, 2007, at 10:39 AM, King Simon-NFHD78 wrote:
> a_table = Table('a', metadata,
> Column('id', Integer, primary_key=True),
> Column('name', String(16)),
> )
>
> b_table = Table('b', metadata,
> Column('id', Integer, primary_key=True),
> Column('name', String(16)),
> )
>
> c_table = Table('c', metadata,
> Column('id', Integer, primary_key=True),
> Column('name', String(16)),
> )
>
> a_b_table = Table('a_b', metadata,
> Column('a_id', Integer, ForeignKey('a.id'),
> primary_key=True),
> Column('b_id', Integer, ForeignKey('b.id'),
> primary_key=True),
> )
> b_c_table = Table('b_c', metadata,
> Column('b_id', Integer, ForeignKey('b.id'),
> primary_key=True),
> Column('c_id', Integer, ForeignKey('c.id'),
> primary_key=True)
> )
>
> class A(object):
> pass
>
> class B(object):
> pass
>
> class C(object):
> pass
>
> mapper(B, b_table)
> mapper(C, c_table)
>
> #############################################################
> # How can I create a mapper on A with a property that gives
> # all the 'C' objects?
> #############################################################
> # This doesn't work - it requires the foreign_keys parameter
> # to be passed, but I don't know what to pass.
> mapper(
> A, a_table,
> properties={'cs': relation(primaryjoin=and_(a_table.c.id ==
> a_b_table.c.a_id,
> a_b_table.c.b_id ==
> b_c_table.c.b_id,
> c_table.c.id ==
> b_c_table.c.c_id),
> viewonly=True,
> )
> }
> )
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