don't check for immutability when setting skip_preloading as it doesn't effect the arel and the arel may already be generated by fresh_when

activerecord/lib/active_record/relation.rb

@@ -8,8 +8,8 @@ class Relation
                             :extending, :unscope]
 
     SINGLE_VALUE_METHODS = [:limit, :offset, :lock, :readonly, :reordering,
-                            :reverse_order, :distinct, :create_with, :skip_query_cache,
-                            :skip_preloading]
+                            :reverse_order, :distinct, :create_with, :skip_query_cache]
+
     CLAUSE_METHODS = [:where, :having, :from]
     INVALID_METHODS_FOR_DELETE_ALL = [:distinct, :group, :having]
 
@@ -19,6 +19,7 @@ class Relation
     include FinderMethods, Calculations, SpawnMethods, QueryMethods, Batches, Explain, Delegation
 
     attr_reader :table, :klass, :loaded, :predicate_builder
+    attr_accessor :skip_preloading_value
     alias :model :klass
     alias :loaded? :loaded
     alias :locked? :lock_value

activerecord/test/cases/relation/mutation_test.rb

@@ -59,7 +59,7 @@ class RelationMutationTest < ActiveRecord::TestCase
       assert_equal [], relation.extending_values
     end
 
-    (Relation::SINGLE_VALUE_METHODS - [:lock, :reordering, :reverse_order, :create_with, :skip_query_cache, :skip_preloading]).each do |method|
+    (Relation::SINGLE_VALUE_METHODS - [:lock, :reordering, :reverse_order, :create_with, :skip_query_cache]).each do |method|
       test "##{method}!" do
         assert relation.public_send("#{method}!", :foo).equal?(relation)
         assert_equal :foo, relation.public_send("#{method}_value")

activerecord/test/cases/relation_test.rb

@@ -315,6 +315,14 @@ def test_update_all_goes_through_normal_type_casting
       assert_equal "type cast from database", UpdateAllTestModel.first.body
     end
 
+    def test_skip_preloading_after_arel_has_been_generated
+      assert_nothing_raised do
+        relation = Comment.all
+        relation.arel
+        relation.skip_preloading!
+      end
+    end
+
     private
 
       def skip_if_sqlite3_version_includes_quoting_bug