Question

With the following tables and mappings

class A:
    def __init__(self):
        self.id = None
        self.b_ids = {}
        self.b_s = {}

class B:
    def __init__ (self):
        self.id = None
        self.a_id = None
        self.a = None

a = Table('t_a', meta,
    Column('id',Integer, autoincrement=True, primary_key=True),
    )

b = Table('t_b', meta,
    Column('id',Integer, autoincrement=True, primary_key=True),
    Column('a_id', Integer, ForeignKey('t_a.id')),
    )

mapper(A, a)
mapper(B, b, properties={'a' : relationship(A, backref="b_s")})

When I load ‘A’ I can get the related ‘B’ objects in the ‘b_s’ property. But What I want is a list of ids of related Bs in the A.b_ids property. Is there a way to do this?

I tried :

mapper(A, a, properties={'b_ids' : 
                         column_property(select(
                                                [b.c.id], 
                                                a.c.id==b.c.a_id))
                         })

But its giving error: ‘ProgrammingError: (ProgrammingError) more than one row returned by a subquery used as an expression’

Answer 1

column_property() is used to add another column to the SELECT statement, such as this:

SELECT a.x, a.y, (SELECT b.id FROM b where a.id=b.a_id) AS b_id FROM a

In SQL, that’s a subquery in the columns clause of the SELECT statement, and it needs to return exactly one row/column for each row – in SQLAlchemy we call this a “scalar select”.

In this case you’re looking for a list, so that’s not embedded into the main columns clause there. That’s why the database returns an error for that. Your mapping already has a list of “b ids” on it, in the form of “b_s”. You just want to extract the “id” from that, and this can be done pretty easily with the association proxy:

from sqlalchemy.ext.associationproxy import association_proxy

class A(object):
    # ... other things
    b_ids = association_proxy('b_s', 'id')

When you access “a.b_ids”, it will look at each entry within “b_s” and extract the “id” attribute, returning a collection that’s compatible with the type of collection of “b_s”, which in this case is a Python list.