On Wed, Jun 18, 2014 at 8:40 AM, Bao Niu <[email protected]> wrote:
> I have a Person class and a Names class, which have a one-to-many
> relationship ( a person can have many names ).
>
> After doing a query like this:
>>
>> p = session.query(Person).filter(Person.birthday > "1992-01-01").one()
>
>
> I'd like to further perform a query on the returned object p to select only
> its names that starts with letter "B".
>
> Currently I am using a very basic way of looping(by the way I'm using
> automap bases, which is cool!) :
>>
>> for n in p.names_collection:
>> if n.startswith('B'):
>> names.append(n)
>
>
> I'm wondering if there is some sqlalchemy-specific ways of querying an
> attributes on a returned object? Just need a hint here, I can do the
> research following the lead. Many thanks.
>
There's a couple of ways of doing this, and which you use really
depends on your use case.
One option is to explicitly query the Names table, using the
"with_parent" method to restrict the results to names belonging to the
correct person:
name_query =
session.query(Names).with_parent(p).filter(Names.name.startswith('B'))
http://docs.sqlalchemy.org/en/rel_0_9/orm/tutorial.html#common-relationship-operators
Another is to configure the relationship as "dynamic":
http://docs.sqlalchemy.org/en/rel_0_9/orm/collections.html#dynamic-relationship
If you do this, Person.names_collection will no longer be a simple
Python list, but instead a special kind of query that is preconfigured
to select rows related to the person instance. You would use it like:
name_query = p.names_collection.filter(Names.name.startswith('B'))
This usually only makes sense for very large collections, since you
can't really eager-load a dynamic relationship.
Hope that helps,
Simon
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