ArgumentError: Polymorphic association does not support to compute class

[mediafinger]

Thank you for maintaining this library 😃
We only encountered it now, after having a composite index in one of our MySQL tables and threw it in the mix. Surprisingly this leads to multiple errors in unrelated classes:

The call to source_reflection.klass will raise the ArgumentError:
Polymorphic association does not support to compute class
when source_reflection.klass is polymorphic.

module ActiveRecord
  module Associations
    module ThroughAssociation
      alias :original_construct_join_attributes :construct_join_attributes

      def construct_join_attributes(*records)
        # CPK
        if source_reflection.klass.composite?
          ensure_mutable

          ids = records.map do |record|
            source_reflection.association_primary_key(reflection.klass).map do |key|
              record.send(key)
            end
          end

          cpk_in_predicate(through_association.scope.klass.arel_table, source_reflection.foreign_key, ids)
        else
          original_construct_join_attributes(*records)
        end
      end
    end
  end
end

This change was discussed here: rails/rails#31877
and implemented here: rails/rails@fb86ecd

For our code base a monkey-patch to change the condition to:
if !source_reflection.polymorphic? && source_reflection.klass.composite?
would suffice, but I doubt this would be a good solution for this gem.

[cfis]

I'm ok with that change. Would you be able to submit a patch with a test case?

[cfis]

After looking at this again, I think your patch is good enough for now and I applied it to the Rails 6.1 and Rails 7 branches. Thanks!