first off, note that this style of relationship is severely limited
with regards to the arrangements of A->B->C. the C's attached to B's
are intertwined with the B's that are attached to A's and it will be
very confusing to manage those relationships independently of each
other. if you are looking for select efficiency, you would probably
be better off just doing normal joins from A->B->C including
association tables if they are many-to-many relationships; SA can
eager load such a relationship in one query.
anyway, the closest SA can get to this is to represent your "R" table
as a relationship object with its own mapper:
class ABCRelation(object):pass
mapper(A, Atable)
mapper(B,Btable)
mapper(C,Ctable)
mapper(ABCRelation, Rtable, primary_key=[R.c.a_id, R.c.b_id,
R.c.c_id], properties={
'as':relation(A, backref='relations'),
'bs':relation(B, backref='relations'),
'cs':relation(C, backref='relations')
}, allow_null_pks=True)
a = A()
b = B()
c = C()
r = ABCRelation()
r.a = a
r.b = b
r.c = c
[session.save(x) for x in [a,b,c,r]]
session.flush()
etc
the good news is that the latest trunk of SQLAlchemy (i.e. release
0.2.9) will even keep it straight if you delete an ABCRelation and
create a new one with the same relationships in one session; it will
turn the DELETE/INSERT into an UPDATE.
On Sep 27, 2006, at 1:10 PM, Martin Kaffanke wrote:
> Hi there!
>
> I don't know if my english is good enaugh to tell you what I want.
>
> I have 3 Tables, lets tell them A, B and C. The relations:
>
> An A kann have some B's and a B can have some C's. But the main
> Select
> will be:
>
> and_(A.c.a_id==B.c.a_id, B.c.b_id==C.c.b_id, C.c.c_id=some_integer)
>
> So now there are two joins.
> As you see, we have two joins. We would have only one by doing
>
> A B C and a R table, where R has all three ID's:
>
> R.c.a_id
> R.c.b_id
> R.c.c_id
>
> A relation table...
>
> and_(A.c.a_id==R.c.a_id, C.c.c_id==some_integer)
>
> would be enaught.
>
> But how can I save data this way with a mapper?
>
> Martin
>
>
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