On Nov 26, 2009, at 2:05 AM, Kalium wrote:
> Hi, I'm coming to grief trying to create a Mapper on the fly.
>
> At first I tried this
>
> class Dynamic(object):
> pass
>
> Dynamic.mapper = mapper(Dynamic, self.j);
>
> self.j is a join statement. This now maps all the columns from the
> tables in the join statement. Which is fine. Except that in the SQL
> query it looks something like "SELECT table1.id AS id, table2.id AS
> id etc etc". So that won't work as the columns don't have alias'.
the join() by itself is automatically disambiguating so you shouldn't have such
an issue:
from sqlalchemy import *
from sqlalchemy.orm import *
m = MetaData(e)
t1 = Table('t1', m, Column('id', Integer, primary_key=True))
t2 = Table('t2', m, Column('id', Integer, primary_key=True))
j = t1.join(t2, t1.c.id==t2.c.id)
# j now has j.c.t1_id, j.c.t2_id
print j.c.t1_id, j.c.t2_id
returns:
t1.id t2.id
now map:
class C(object):
pass
mapper(C, j)
print create_session().query(C)
returns:
SELECT t1.id AS t1_id, t2.id AS t2_id
FROM t1 JOIN t2 ON t1.id = t2.id
the map of C will have a single attribute "id" on it, which is based on that
name. in this case, that's fine since the two "id" columns are equated, and
you'd want them to be under one attribute. If you didn't, you can set up the
mapping explicitly with the names you prefer:
m = MetaData(create_engine('sqlite://'))
t1 = Table('t1', m, Column('id', Integer, primary_key=True), Column('data',
String))
t2 = Table('t2', m, Column('id', Integer, primary_key=True), Column('data',
String))
m.create_all()
j = t1.join(t2, t1.c.id==t2.c.id)
class C(object):
pass
mapper(C, j, properties={
't1data':t1.c.data,
't2data':t2.c.data
})
s = sessionmaker()()
c1 = C()
c1.t1data='foo'
c1.t2data='bar'
s.add(c1)
s.commit()
print s.execute("select * from t1 join t2 on t1.id=t2.id", mapper=C).fetchall()
returns:
[(1, u'foo', 1, u'bar')]
above, the mapper for "C" has a single attribute "id" representing t1.c.id and
t2.c.id, but separate "t1data" and "t2data" attributes representing t1.c.data
and t2.c.data, respectively.
>
> Next I tried something like this
>
> Dynamic.mapper = mapper(Dynamic,select(fields_to_select,from_obj=
> [self.j]),primary_key=[table1.c.id,table2.c.id,table3.c.id])
here, you've mapped to a select() construct - that is your selectable. The
mistake here is that you used columns for "primary_key" which are from your
tables - which are represented within the internal FROM clause of your select()
but are not the columns it publically exposes in its columns clause. Any
columns you specify to primary_key must be in terms of the mapped selectable's
"exported" columns clause, i.e. myselect.c.table1_id, myselect.c.table2_id,
myselect.c.table3_id. Specifying use_labels on your select() construct will
allow the column names it exports in its "c" collection to be prepended with
the table name, the same way that the join() does automatically.
But also, if your select() already has all the PK fields from the three tables
represented, the primary key columns will automatically be pulled out in the
mapping so there is probably no need for the primary_key argument.
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