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

There was a bug in the handling of duplicates when
assigning (replacing) associated records, which made the result
dependent on whether a given record was associated already before
being assigned anew. E.g.

    post.people = [person, person]
    post.people.count
    # => 2

while

    post.people = [person]
    post.people = [person, person]
    post.people.count
    # => 1

This change adds a test to provoke the former incorrect behavior, and
fixes it.

Cause of the bug was the handling of record collections as sets, and
using `-` (difference) and `&` (union) operations on them
indiscriminately. This temporary conversion to sets would eliminate
duplicates.

The fix is to decorate record collections for these operations, and
only for the `has_many :through` case. It is done by counting
occurrences, and use the record together with the occurrence number as
element, in order to make them work well in sets. Given

    a, b = *Person.all

then the collection used for finding the difference or union of
records would be internally changed from

     [a, b, a]

to

     [[a, 1], [b, 1], [a, 2]]

for these operations. So a first occurrence and a second occurrence
would be distinguishable, which is all that is necessary for this
task.

Fixes #33942.

1 files changed, 10 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/has_many_through_associations_test.rb b/activerecord/test/cases/associations/has_many_through_associations_test.rb
index 7b405c74c4..cf514957ca 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/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)]