Fix type cast on group sum with custom expression

For PG adapters with custom expression and grouped result
of aggregate functions have not found correct column type
for it. Extract column type from query result.

Closes: #13230

activerecord/CHANGELOG.md

+*   Use the right column to type cast grouped calculations with custom expressions.
+
+    Fixes: #13230
+
+    Example:
+
+            # Before
+            Account.group(:firm_name).sum('0.01 * credit_limit')
+            # => { '37signals' => '0.5' }
+
+            # After
+            Account.group(:firm_name).sum('0.01 * credit_limit')
+            # => { '37signals' => 0.5 }
+
+    *Paul Nikitochkin*
+
 *   Polymorphic `belongs_to` associations with the `touch: true` option set update the timestamps of
     the old and new owner correctly when moved between owners of different types.
 

activerecord/lib/active_record/relation/calculations.rb

@@ -311,7 +311,9 @@ def execute_grouped_calculation(operation, column_name, distinct) #:nodoc:
         }
         key = key.first if key.size == 1
         key = key_records[key] if associated
-        [key, type_cast_calculated_value(row[aggregate_alias], column_for(column_name), operation)]
+
+        column_type = calculated_data.column_types.fetch(aggregate_alias) { column_for(column_name) }
+        [key, type_cast_calculated_value(row[aggregate_alias], column_type, operation)]
       end]
     end
 

activerecord/test/cases/calculations_test.rb

@@ -211,6 +211,10 @@ def test_sum_should_return_valid_values_for_decimals
     assert_equal 19.83, NumericData.sum(:bank_balance)
   end
 
+  def test_should_return_type_casted_values_with_group_and_expression
+    assert_equal 0.5, Account.group(:firm_name).sum('0.01 * credit_limit')['37signals']
+  end
+
   def test_should_group_by_summed_field_with_conditions
     c = Account.where('firm_id > 1').group(:firm_id).sum(:credit_limit)
     assert_nil        c[1]