To optimize a query I'm attempting to defer columns from joined tables
I know I'm not going to need. The columns from joined tables to defer
varies from query to query so I can't specify this when setting up my
object/table mappings.
I'm able to do this just fine for columns of the primary table like
this: query = query.option(defer("x")).
It appears there are two ways to do the same for joined columns:
1] When specifying entities
query = a_session.query(P, S1.x, S2.y, S1Alias.x)...
2] Using path to the joined column:
query = query.option(defer("a.s1_relation.col1"), defer
("a.s1_alias_relation.col2"),...)
1
Seemed easy enough but i ran into problems when using this in
conjunction with query.one(): it would raise exceptions that the
result set had more than one row. I really just want to pull the
entity object (preloaded with relevant column values, some from
related tables) and then extract that to a dictionary. So pulling
S1.x, S2.y, S1Alias.x into the result, making it a tuple, seems a
little strange.
2
Simply doesn't work for me (i.e. the columns i'm deferring are clearly
in the echoed sql).
I also tried passing instrumented object attributes to defers like
this: query = query.option(defer(S1.y), defer(S1Alias.x)) but that
failed with an exception about e.g. S1 not being found in the mapper.
I tried adding them to the query like this: query.add_entity(S1) but
that didn't help.
Apart from the unwanted columns the emitted SQL is what i expect and
looks like:
SELECT <bunch of columns>
FROM parent
LEFT OUTER JOIN parent_child AS series_child_1 ON series.id =
series_child_1.series_id
LEFT OUTER JOIN child ON series_child_1.child_id = child.id
<-- primary child
INNER JOIN child AS child_1 ON child_1.id =
series.parent_child_id
WHERE series.id = 1000 AND child.name LIKE 'prefix%'
Am i missing something? Is there a way to specify joined column defers
during query construction? Do i need to write a custom MapperProperty
to do this?
Thanks,
M
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