I think the join.select(columns, whereclasue) part here is wrong, the select()
method unfortunately does not have that calling signature (I wish it did, but
there's a lot of legacy there).
You can get the columns you want more explicitly, select([c1, c2, c3,
..]).select_from(my_join).
The exception raised here is probably sqlalchemy being surprised by the list of
columns being sent (the error message would do better to report on what it
sees).
On Sep 4, 2014, at 1:11 AM, apoorv.x.kulshres...@barclays.com wrote:
Hi All,
I am joining 3 tables in SQLAlchemy Core and selecting all columns as follows:
rows = self.db.execute(self.execs.join(
self.orders.join(self.instruments)
).select(whereClause)).reduce_columns())
It works well but if I want to select a subset of columns:
reqdCols = [order.c.id, exec.c.last_modified, instruments.type]
rows = self.db.execute(self.execs.join(
self.orders.join(self.instruments)
).select(reqdCols, whereClause)).reduce_columns())
It doesn't works and gives following error:
Traceback (most recent call last):
File stdin, line 1, in module
File
/apps/qtdist/pkgs/pub/cpython/python-2.7.6/lib/python2.7/site-packages/sqlalchemy/sql/selectable.py,
line 807, in select
return Select(collist, whereclause, from_obj=[self], **kwargs)
File
/apps/qtdist/pkgs/pub/cpython/python-2.7.6/lib/python2.7/site-packages/sqlalchemy/sql/selectable.py,
line 2219, in __init__
whereclause).self_group(against=operators._asbool)
File
/apps/qtdist/pkgs/pub/cpython/python-2.7.6/lib/python2.7/site-packages/sqlalchemy/sql/elements.py,
line 3438, in _literal_as_text
SQL expression object or string expected.
sqlalchemy.exc.ArgumentError: SQL expression object or string expected.
Alternative would be to use select instead of Join.select and make it
implicit join with where clauses:
joinConditions = (orders.c.colx == execs.colx) (execs.c.coly ==
instruments.c.coly)
select(reqdCols).where(and_(whereClause, joinConditions)
But I would prefer explicit join over implicit for performance reasons. Is
there any way to select subset of columns using explicit joins?
Regards,
Apoorv
PS: I have post the same in Stack Overflow Question:
http://stackoverflow.com/questions/25656935/selecting-columns-from-mulitple-table-join-in-sqlalchemy-core#
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