The primary key is always initialized in the @attributes hash to nil (unless

another value has been specified).

5 files changed, 12 insertions, 7 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index a458f9e6e4..09431e38f5 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,8 @@
 ## Rails 4.0.0 (unreleased) ##
 
+* The primary key is always initialized in the @attributes hash to nil (unless
+  another value has been specified).
+
 *   In previous releases, the following would generate a single query with
     an `OUTER JOIN comments`, rather than two separate queries:
 
diff --git a/activerecord/lib/active_record/attribute_methods/read.rb b/activerecord/lib/active_record/attribute_methods/read.rb
index 3549cbb090..c129dc8c52 100644
--- a/activerecord/lib/active_record/attribute_methods/read.rb
+++ b/activerecord/lib/active_record/attribute_methods/read.rb
@@ -92,11 +92,7 @@ module ActiveRecord
         end
 
         def internal_attribute_access_code(attr_name, cast_code)
-          if attr_name == primary_key
-            access_code = "v = @attributes[attr_name];"
-          else
-            access_code = "v = @attributes.fetch(attr_name) { missing_attribute(attr_name, caller) };"
-          end
+          access_code = "v = @attributes.fetch(attr_name) { missing_attribute(attr_name, caller) };"
 
           access_code << "v && #{cast_code};"
 
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index adba3f710c..a774af6024 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -202,6 +202,7 @@ module ActiveRecord
       cloned_attributes.delete(self.class.primary_key)
 
       @attributes = cloned_attributes
+      @attributes[self.class.primary_key] = nil
 
       run_callbacks(:initialize) if _initialize_callbacks.any?
 
@@ -326,6 +327,10 @@ module ActiveRecord
     end
 
     def init_internals
+      pk = self.class.primary_key
+
+      @attributes[pk] = nil unless @attributes.key?(pk)
+
       @relation               = nil
       @aggregation_cache      = {}
       @association_cache      = {}
diff --git a/activerecord/test/cases/associations/has_many_through_associations_test.rb b/activerecord/test/cases/associations/has_many_through_associations_test.rb
index 4612bc2618..9cc09194dc 100644
--- a/activerecord/test/cases/associations/has_many_through_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_through_associations_test.rb
@@ -738,7 +738,7 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
 
   def test_select_chosen_fields_only
     author = authors(:david)
-    assert_equal ['body'], author.comments.select('comments.body').first.attributes.keys
+    assert_equal ['body', 'id'].sort, author.comments.select('comments.body').first.attributes.keys.sort
   end
 
   def test_get_has_many_through_belongs_to_ids_with_conditions
diff --git a/activerecord/test/cases/serialization_test.rb b/activerecord/test/cases/serialization_test.rb
index 61b04b3e37..31ba99a8ff 100644
--- a/activerecord/test/cases/serialization_test.rb
+++ b/activerecord/test/cases/serialization_test.rb
@@ -13,7 +13,8 @@ class SerializationTest < ActiveRecord::TestCase
       :created_at     => Time.utc(2006, 8, 1),
       :awesome        => false,
       :preferences    => { :gem => '<strong>ruby</strong>' },
-      :alternative_id => nil
+      :alternative_id => nil,
+      :id             => nil
     }
   end