Correct through associations using scopes

The changes introduced to through associations in c80487eb were quite
interesting. Changing `relation.merge!(scope)` to `relation =
relation.merge(scope)` should in theory never cause any changes in
behavior. The subtle breakage led to a surprising conclusion.

The old code wasn't doing anything! Since `merge!` calls
`instance_exec` when given a proc, and most scopes will look something
like `has_many :foos, -> { where(foo: :bar) }`, if we're not capturing
the return value, it's a no-op. However, removing the `merge` causes
`unscope` to break.

While we're merging in the rest of the chain elsewhere, we were never
merging in `unscope` values, causing a breakage on associations where a
default scope was being unscoped in an association scope (yuk!). This is
subtly related to #20722, since it appears we were previously relying on
this mutability.

Fixes #20721.
Fixes #20727.

1 files changed, 7 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 1d545af5a5..c34387f06d 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -1158,4 +1158,11 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     post_through = organization.posts.build
     assert_equal post_direct.author_id, post_through.author_id
   end
+
+  def test_has_many_through_with_scope_that_should_not_be_fully_merged
+    Club.has_many :distinct_memberships, -> { distinct }, class_name: "Membership"
+    Club.has_many :special_favourites, through: :distinct_memberships, source: :member
+
+    assert_nil Club.new.special_favourites.distinct_value
+  end
 end