Fix sum with qualified name on loaded relation

Since rails#53064, calling `sum` on a loaded relation raised
`ActiveRecord::UnmodifiableRelation`.

Author: cgunther

activerecord/lib/active_record/relation/calculations.rb

@@ -410,6 +410,18 @@ def async_ids
       async.ids
     end
 
+    protected
+      def aggregate_column(column_name)
+        case column_name
+        when Arel::Expressions
+          column_name
+        when :all
+          Arel.star
+        else
+          arel_column(column_name)
+        end
+      end
+
     private
       def all_attributes?(column_names)
         (column_names.map(&:to_s) - model.attribute_names - model.attribute_aliases.keys).empty?
@@ -457,17 +469,6 @@ def possible_aggregation?(column_names)
         end
       end
 
-      def aggregate_column(column_name)
-        case column_name
-        when Arel::Expressions
-          column_name
-        when :all
-          Arel.star
-        else
-          arel_column(column_name)
-        end
-      end
-
       def operation_over_aggregate_column(column, operation, distinct)
         operation == "count" ? column.count(distinct) : column.public_send(operation)
       end
@@ -483,7 +484,7 @@ def execute_simple_calculation(operation, column_name, distinct) # :nodoc:
           # PostgreSQL doesn't like ORDER BY when there are no GROUP BY
           relation = unscope(:order).distinct!(false)
 
-          column = aggregate_column(column_name)
+          column = relation.aggregate_column(column_name)
           select_value = operation_over_aggregate_column(column, operation, distinct)
           select_value.distinct = true if operation == "sum" && distinct
 
@@ -532,7 +533,9 @@ def execute_grouped_calculation(operation, column_name, distinct) # :nodoc:
           }
           group_columns = group_aliases.zip(group_fields)
 
-          column = aggregate_column(column_name)
+          relation = except(:group).distinct!(false)
+
+          column = relation.aggregate_column(column_name)
           column_alias = column_alias_tracker.alias_for("#{operation} #{column_name.to_s.downcase}")
           select_value = operation_over_aggregate_column(column, operation, distinct)
           select_value.as(model.adapter_class.quote_column_name(column_alias))
@@ -549,7 +552,6 @@ def execute_grouped_calculation(operation, column_name, distinct) # :nodoc:
             end
           }
 
-          relation = except(:group).distinct!(false)
           relation.group_values  = group_fields
           relation.select_values = select_values
 
@@ -666,7 +668,7 @@ def build_count_subquery(relation, column_name, distinct)
           relation.unscope!(:order)
         else
           column_alias = Arel.sql("count_column")
-          relation.select_values = [ aggregate_column(column_name).as(column_alias) ]
+          relation.select_values = [ relation.aggregate_column(column_name).as(column_alias) ]
         end
 
         subquery_alias = Arel.sql("subquery_for_count", retryable: true)

activerecord/test/cases/calculations_test.rb

@@ -41,6 +41,18 @@ def test_should_sum_arel_attribute
     assert_async_equal 318, Account.async_sum(Account.arel_table[:credit_limit])
   end
 
+  def test_should_sum_with_qualified_name_on_loaded
+    accounts = Account.all
+
+    assert_not_predicate accounts, :loaded?
+    assert_equal 318, accounts.sum("accounts.credit_limit")
+
+    accounts.load
+
+    assert_predicate accounts, :loaded?
+    assert_equal 318, accounts.sum("accounts.credit_limit")
+  end
+
   def test_should_average_field
     assert_equal 53.0, Account.average(:credit_limit)
     assert_async_equal 53.0, Account.async_average(:credit_limit)