rails: AR::Relation::Calculations.count breaks relations with group and select

Goal: get a count of rows from a query that includes pseudo columns and grouping.

When counting with grouping in the relation, ActiveRecord tries to return group counts instead of a total count and drops the SELECT on the floor. If there are pseudo-columns in the SELECT for grouping, this breaks the whole query.

Also, if it even worked, this would return a hash instead of a Fixnum. There doesn’t seem to be a way to get the total of returned rows, ignoring the grouping.

count should be usable to return a numeric count even when grouping is needed for the query.
relation.count and relation.all.count should have the same semantics.
count should work directly on any relation, so the relation can still be used as a chainable scope.

Suggestion for Rails 4: The Calculations#count method is too ambiguous and assumes too much. Change Calculations#count method to only return groups if a :group option is passed directly to the count method, i.e. do not trigger grouped counts based on other group_values in the relation. That, or split #count and #count_grouped into separate methods. This might make it easier to do the right thing with the query builder.

A simple first step might be to flag the relation so that it conditionally delegates .count to .all.count when the above situation occurs. Thoughts?

About this issue

Original URL
State: closed
Created 12 years ago
Comments: 21 (19 by maintainers)

Most upvoted comments

I’m also having this problem when paginating the results of a query with ARel in a .group call (supposedly, Kaminari.paginate calls count under the hood).

Here’s some interesting facts:

On Rails 3.2.2

> User.group(User.arel_table[:id]).size
# NoMethodError: undefined method `to_sym' for #<Arel::Attributes::Attribute:0xb0007c0>
from /active_record/relation/calculations.rb:248:in `execute_grouped_calculation'
> User.group('users.id').size
#  SELECT COUNT(*) AS count_all, users.id AS users_id FROM `users` GROUP BY users.id
=> {1=>1}

On Rails 3.2.7

> User.group(User.arel_table[:id]).size
# SELECT COUNT(*) AS count_all, `users`.`id` AS struct_arel_attributes_attribute_relation_arel_table_0xadfe044_name_users_engine_user_id_integer_name_string_created_at_datetime_updated_at_datetime_columns_nil_aliases_table_alias_nil_primary_key_nil_name_id FROM `users` GROUP BY `users`.`id`
=> {1=>1}

Notice the funny column name in the last query. Looks definitely like somewhere obj.to_s.underscore is called on an object which doesn’t expect it.

As a workaround, I separately store the scope for pagination(without grouping), and chain a .group call only when fetching the data itself. But I suppose, pretty many people use pagination so this is a common use case and should be addressed.

RKushnir on Aug 13, 2012