Merge pull request #33954 from febeling/inconsistent-assignment-has-many-through-33942

Fix handling of duplicates for `replace` on has_many-through

activerecord/lib/active_record/associations/collection_association.rb

@@ -413,9 +413,9 @@ module ActiveRecord
         end
 
         def replace_records(new_target, original_target)
-          delete(target - new_target)
+          delete(difference(target, new_target))
 
-          unless concat(new_target - target)
+          unless concat(difference(new_target, target))
             @target = original_target
             raise RecordNotSaved, "Failed to replace #{reflection.name} because one or more of the " \
                                   "new records could not be saved."
@@ -425,7 +425,7 @@ module ActiveRecord
         end
 
         def replace_common_records_in_memory(new_target, original_target)
-          common_records = new_target & original_target
+          common_records = intersection(new_target, original_target)
           common_records.each do |record|
             skip_callbacks = true
             replace_on_target(record, @target.index(record), skip_callbacks)

activerecord/lib/active_record/associations/has_many_association.rb

@@ -130,6 +130,14 @@ module ActiveRecord
           end
           saved_successfully
         end
+
+        def difference(a, b)
+          a - b
+        end
+
+        def intersection(a, b)
+          a & b
+        end
     end
   end
 end

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -163,6 +163,28 @@ module ActiveRecord
           end
         end
 
+        def difference(a, b)
+          distribution = distribution(b)
+
+          a.reject { |record| mark_occurrence(distribution, record) }
+        end
+
+        def intersection(a, b)
+          distribution = distribution(b)
+
+          a.select { |record| mark_occurrence(distribution, record) }
+        end
+
+        def mark_occurrence(distribution, record)
+          distribution[record] > 0 && distribution[record] -= 1
+        end
+
+        def distribution(array)
+          array.each_with_object(Hash.new(0)) do |record, distribution|
+            distribution[record] += 1
+          end
+        end
+
         def through_records_for(record)
           attributes = construct_join_attributes(record)
           candidates = Array.wrap(through_association.target)

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -586,6 +586,16 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert_not_includes posts(:welcome).reload.people.reload, people(:michael)
   end
 
+  def test_replace_association_with_duplicates
+    post   = posts(:thinking)
+    person = people(:david)
+
+    assert_difference "post.people.count", 2 do
+      post.people = [person]
+      post.people = [person, person]
+    end
+  end
+
   def test_replace_order_is_preserved
     posts(:welcome).people.clear
     posts(:welcome).people = [people(:david), people(:michael)]