在Rails Postgres上按字段分组结果

Question

I'm studying rails at the moment and i'm trying to group results of a query by field .

the query is being created by:
@orders = Order.includes(:product).where(user_id: current_user.id, paid: 0).group(:product_id)

It works fine on sqlite (dev env) but it throws a fatal error while on production (postgreSQL).

Error is:

ActionView::Template::Error (PG::GroupingError: ERROR:  column "orders.id" must appear in the GROUP BY clause or be used in an aggregate function

Hmm.. what am i doing wrong here? Thanks

Answer 1

If you want to group in postgres, you need to either directly select that column or you need to call an aggregate function that uses this column. Count is such an aggregate function and you can achieve that in Rails via

@product_counts = Order.where(user: current_user, paid: 0).group(:product_id).count

Postgres automatically executes Count on the column you grouped by, which is product_id in this case. This will give you a hash like this:

{ product_id => count }

For more information, see here: https://www.tutorialspoint.com/postgresql/postgresql_group_by.htm