Use Enumerable#sum on ActiveRecord::Relation when a block is given

This matches our behavior in other cases where useful enumerable methods
might have a different definition in `Relation`. Wanting to actually
enumerate over the records in this case is completely reasonable, and
wanting `.sum` is reasonable for the same reason it is on `Enumerable`
in the first place.

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Use `Enumerable#sum` in `ActiveRecord::Relation` if a block is given.
+
+    *Sean Griffin*
+
 *   Let `WITH` queries (Common Table Expressions) be explainable.
 
     *Vladimir Kochnev*

activerecord/lib/active_record/relation/calculations.rb

@@ -70,8 +70,12 @@ def maximum(column_name)
     # +calculate+ for examples with options.
     #
     #   Person.sum(:age) # => 4562
-    def sum(*args)
-      calculate(:sum, *args)
+    def sum(*args, &block)
+      if block_given?
+        to_a.sum(&block)
+      else
+        calculate(:sum, *args)
+      end
     end
 
     # This calculates aggregate values in the given column. Methods for count, sum, average,

activerecord/test/cases/calculations_test.rb

@@ -671,4 +671,14 @@ def test_should_reference_correct_aliases_while_joining_tables_of_has_many_throu
       developer.ratings.includes(comment: :post).where(posts: { id: 1 }).count
     end
   end
+
+  def test_sum_uses_enumerable_version_when_block_is_given
+    block_called = false
+    relation = Client.all.load
+
+    assert_no_queries do
+      assert_equal 0, relation.sum { block_called = true; 0 }
+    end
+    assert block_called
+  end
 end