Fix predicate builder for polymorphic models referencing models with composite primary keys

[fatkodima]

Fixes #50318.

I needed to create 2 models for the test case, because there is no currently Cpk model where both primary key columns are not "id". When one of them is named "id", we do not get the error as in the linked issue

rails/activerecord/lib/active_record/reflection.rb

Lines 875 to 879 in 5c4a7a7

	elsif (klass || self.klass).composite_primary_key?
	# If klass has composite primary key of shape [:<tenant_key>, :id], infer primary_key as :id
	primary_key = (klass || self.klass).primary_key
	primary_key.include?("id") ? "id" : primary_key
	else

[jackozi]

Maybe it's better to move the primary key assignment inside the if clause so that it's not called when hitting the else clause.

[fatkodima]

Yeah, it was for DRYing the code, but turned out this is not working, thanks.

[htcarr3]

Is it possible to merge this fix into 7-1-stable (similar to #50412) ?

[fatkodima]

Opened a backport PR.