Delegate all calculations to the scope.

So that the scope may be a NullRelation and return a result without
executing a query.

Fixes #7928

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -28,6 +28,8 @@ module Associations
     # is computed directly through SQL and does not trigger by itself the
     # instantiation of the actual post records.
     class CollectionProxy < Relation
+      delegate *ActiveRecord::Calculations.public_instance_methods, to: :scope
+
       def initialize(association) #:nodoc:
         @association = association
         super association.klass, association.klass.arel_table
@@ -965,10 +967,6 @@ def reload
         proxy_association.reload
         self
       end
-
-      def pluck(column_names)
-        scope.pluck(column_names)
-      end
     end
   end
 end

activerecord/lib/active_record/null_relation.rb

@@ -50,6 +50,10 @@ def count(*)
       0
     end
 
+    def sum(*)
+      0
+    end
+
     def calculate(_operation, _column_name, _options = {})
       nil
     end

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1656,6 +1656,7 @@ def test_collection_association_with_private_kernel_method
       assert_equal [], post.comments
       assert_equal [], post.comments.where(body: 'omg')
       assert_equal [], post.comments.pluck(:body)
+      assert_equal 0,  post.comments.sum(:id)
     end
   end
 end