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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