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.

0 files changed, 0 insertions, 0 deletions