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.

activerecord/CHANGELOG.md

@@ -1,3 +1,11 @@
+*   Fix through associations using scopes having the scope merged multiple
+    times.
+
+    Fixes #20721.
+    Fixes #20727.
+
+    *Sean Griffin*
+
 *   `ActiveRecord::Base.dump_schema_after_migration` applies migration tasks
     other than `db:migrate`. (eg. `db:rollback`, `db:migrate:dup`, ...)
 

activerecord/lib/active_record/associations/association_scope.rb

@@ -149,6 +149,7 @@ def add_constraints(scope, owner, association_klass, refl, chain_head, chain_tai
 
             scope.where_clause += item.where_clause
             scope.order_values |= item.order_values
+            scope.unscope!(*item.unscope_values)
           end
 
           reflection = reflection.next

activerecord/lib/active_record/associations/through_association.rb

@@ -15,12 +15,6 @@ def target_scope
           scope = super
           reflection.chain.drop(1).each do |reflection|
             relation = reflection.klass.all
-
-            reflection_scope = reflection.scope
-            if reflection_scope && reflection_scope.arity.zero?
-              relation = relation.merge(reflection_scope)
-            end
-
             scope.merge!(
               relation.except(:select, :create_with, :includes, :preload, :joins, :eager_load)
             )

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -1158,4 +1158,11 @@ def test_build_for_has_many_through_association
     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