Ensure sum honors distinct on has_many through

When using a has_many through relation and then summing an attribute
the distinct was not being used. This will ensure that when summing
an attribute, the number is only used once when distinct has been used.

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Ensure `sum` honors `distinct` on `has_many :through` associations
+
+    Fixes #16791
+
+    *Aaron Wortham
+
 *   When using `Relation#or`, extract the common conditions and put them before the OR condition.
 
     *Maxime Handfield Lapointe*

activerecord/lib/active_record/relation/calculations.rb

@@ -259,6 +259,9 @@ module ActiveRecord
           column = aggregate_column(column_name)
 
           select_value = operation_over_aggregate_column(column, operation, distinct)
+          if operation == "sum" && distinct
+            select_value.distinct = true
+          end
 
           column_alias = select_value.alias
           column_alias ||= @klass.connection.column_name_for_operation(operation, select_value)

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -1139,6 +1139,32 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert_equal ["parrot", "bulbul"], owner.toys.map { |r| r.pet.name }
   end
 
+  def test_has_many_through_associations_sum_on_columns
+    post1 = Post.create(title: "active", body: "sample")
+    post2 = Post.create(title: "inactive", body: "sample")
+
+    person1 = Person.create(first_name: "aaron", followers_count: 1)
+    person2 = Person.create(first_name: "schmit", followers_count: 2)
+    person3 = Person.create(first_name: "bill", followers_count: 3)
+    person4 = Person.create(first_name: "cal", followers_count: 4)
+
+    Reader.create(post_id: post1.id, person_id: person1.id)
+    Reader.create(post_id: post1.id, person_id: person2.id)
+    Reader.create(post_id: post1.id, person_id: person3.id)
+    Reader.create(post_id: post1.id, person_id: person4.id)
+
+    Reader.create(post_id: post2.id, person_id: person1.id)
+    Reader.create(post_id: post2.id, person_id: person2.id)
+    Reader.create(post_id: post2.id, person_id: person3.id)
+    Reader.create(post_id: post2.id, person_id: person4.id)
+
+    active_persons = Person.joins(:readers).joins(:posts).distinct(true).where("posts.title" => "active")
+
+    assert_equal active_persons.map(&:followers_count).reduce(:+), 10
+    assert_equal active_persons.sum(:followers_count), 10
+    assert_equal active_persons.sum(:followers_count), active_persons.map(&:followers_count).reduce(:+)
+  end
+
   def test_has_many_through_associations_on_new_records_use_null_relations
     person = Person.new