添加外键表时，Sqlalchemy添加不需要的列以进行查询

Question

In a Sqlalchemy query, you can select specific rows like this:

q = session.query(Equipment.name)

That will return one column for each row. You can add more column descriptors from model. The value of len(q.column_descriptions) is one in this case.

However, as soon as you add a column that is a foreign key you suddenly get all the columns of the table, even though I didn't ask for them.

q = dbsession.query(models.Equipment.name, models.Equipment.model)

This should be two columns, but now len(q.column_descriptions) is 15 (for this table, that's all of them plus the addidtional one).

The query (q.statement) ends up looking like this:

SELECT public.equipment.name, public.equipment.id, public.equipment.model_id, public.equipment.serno, public.equipment.location_id, public.equipment.sublocation, public.equipment.addeddate, public.equipment.comments, public.equipment.language_id, public.equipment.owner_id, public.equipment.vendor_id, public.equipment.account_id, public.equipment.parent_id, public.equipment.active 
FROM public.equipment

I think this is a bug in Sqlalchemy, but maybe I'm doing something wrong and somebody here might know what it is.

Answer 1

"name" is a column. "model" is not - its a relationship() (based on the observed behavior above). The current default __clause_element__() for relationship is the full table so that's why all columns get spit out. As you noted in ticket #1328 this is related, but the proposal there is that you'd be getting back ('name', SomeModelObject), so not a foreign key column there either. The difference between "foreign key" and "relationship" is different in SQLAlchemy, in constrast to other ORM tools which may conceal the fact that "foreign key" columns exist.

To query for the FK column only, you want to say query(Equipment.name, Equipment.model_id), or whatever the name of the foreign key column that references "model" is. If you're using Elixir then you may have to dig into their docs to see what name they use.