Fix association with extension issues

This fixes the following issues.

* `association_scope` doesn't include `default_scope`. Should use `scope` instead.
* We can't use `method_missing` for customizing existing method.
* We can't use `relation_delegate_class` for sharing extensions. Should extend per association.

3 files changed, 14 insertions, 22 deletions

diff --git a/activerecord/lib/active_record/associations/association.rb b/activerecord/lib/active_record/associations/association.rb
index 1cb2b2d7c6..e42162c740 100644
--- a/activerecord/lib/active_record/associations/association.rb
+++ b/activerecord/lib/active_record/associations/association.rb
@@ -133,6 +133,16 @@ module ActiveRecord
         AssociationRelation.create(klass, klass.arel_table, klass.predicate_builder, self).merge!(klass.all)
       end
 
+      def extensions
+        extensions = klass.all.extensions | reflection.extensions
+
+        if scope = reflection.scope
+          extensions |= klass.unscoped.instance_exec(owner, &scope).extensions
+        end
+
+        extensions
+      end
+
       # Loads the \target if needed and returns it.
       #
       # This method is abstract in the sense that it relies on +find_target+,
diff --git a/activerecord/lib/active_record/associations/association_scope.rb b/activerecord/lib/active_record/associations/association_scope.rb
index 120d75416c..1593b94f0c 100644
--- a/activerecord/lib/active_record/associations/association_scope.rb
+++ b/activerecord/lib/active_record/associations/association_scope.rb
@@ -24,7 +24,7 @@ module ActiveRecord
         alias_tracker = AliasTracker.create connection, association.klass.table_name, klass.type_caster
         chain_head, chain_tail = get_chain(reflection, association, alias_tracker)
 
-        scope.extending! Array(reflection.options[:extend])
+        scope.extending! reflection.extensions
         add_constraints(scope, owner, reflection, chain_head, chain_tail)
       end
 
diff --git a/activerecord/lib/active_record/associations/collection_proxy.rb b/activerecord/lib/active_record/associations/collection_proxy.rb
index 74a4d515c2..89cfa07be3 100644
--- a/activerecord/lib/active_record/associations/collection_proxy.rb
+++ b/activerecord/lib/active_record/associations/collection_proxy.rb
@@ -31,6 +31,9 @@ module ActiveRecord
       def initialize(klass, association) #:nodoc:
         @association = association
         super klass, klass.arel_table, klass.predicate_builder
+
+        extensions = association.extensions
+        extend(*extensions) if extensions.any?
       end
 
       def target
@@ -1121,19 +1124,6 @@ module ActiveRecord
 
       delegate(*delegate_methods, to: :scope)
 
-      module DelegateExtending # :nodoc:
-        private
-          def method_missing(method, *args, &block)
-            extending_values = association_scope.extending_values
-            if extending_values.any? && (extending_values - self.class.included_modules).any?
-              self.class.include(*extending_values)
-              public_send(method, *args, &block)
-            else
-              super
-            end
-          end
-      end
-
       private
 
         def find_nth_with_limit(index, limit)
@@ -1154,17 +1144,9 @@ module ActiveRecord
           @association.find_from_target?
         end
 
-        def association_scope
-          @association.association_scope
-        end
-
         def exec_queries
           load_target
         end
-
-        def respond_to_missing?(method, _)
-          association_scope.respond_to?(method) || super
-        end
     end
   end
 end