Question

I’ve got several function where I need to do a one-to-many join, using count(), group_by, and order_by. I’m using the sqlalchemy.select function to produce a query that will return me a set of id’s, which I then iterate over to do an ORM select on the individual records. What I’m wondering is if there is a way to do what I need using the ORM in a single query so that I can avoid having to do the iteration.

Here’s an example of what I’m doing now. In this case the entities are Location and Guide, mapped one-to-many. I’m trying get a list of the top locations sorted by how many guides they are related to.

def popular_world_cities(self):     query = select([locations.c.id, func.count(Guide.location_id).label('count')],                    from_obj=[locations, guides],                    whereclause='guides.location_id = locations.id AND (locations.type = 'city' OR locations.type = 'custom')',                    group_by=[Location.id],                    order_by='count desc',                    limit=10)     return map(lambda x: meta.Session.query(Location).filter_by(id=x[0]).first(), meta.engine.execute(query).fetchall()) 

Solution

I’ve found the best way to do this. Simply supply a from_statement instead of a filter_by or some such. Like so:

meta.Session.query(Location).from_statement(query).all() 

Answer 1

I’ve found the best way to do this. Simply supply a from_statement instead of a filter_by or some such. Like so:

meta.Session.query(Location).from_statement(query).all()