Ensure first! and friends work on loaded associations

Fixes #18237

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Ensure `first!` and friends work on loaded associations.
+
+    Fixes #18237.
+
+    *Sean Griffin*
+
 *   `eager_load` preserves readonly flag for associations.
 
     Closes #15853.

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -29,6 +29,7 @@ module Associations
     # instantiation of the actual post records.
     class CollectionProxy < Relation
       delegate(*(ActiveRecord::Calculations.public_instance_methods - [:count]), to: :scope)
+      delegate :find_nth, to: :scope
 
       def initialize(klass, association) #:nodoc:
         @association = association

activerecord/test/cases/associations_test.rb

@@ -264,6 +264,11 @@ def test_scoped_allows_conditions
     end
   end
 
+  test "first! works on loaded associations" do
+    david = authors(:david)
+    assert_equal david.posts.first, david.posts.reload.first!
+  end
+
   def test_reset_unloads_target
     david = authors(:david)
     david.posts.reload