diff --git a/activemodel/lib/active_model/attribute_methods.rb b/activemodel/lib/active_model/attribute_methods.rb
index fc5f5c4c66e4d22d7105364333f5d999bd98ea63..33454123cece7f07a6f3005bfe67128a701fb316 100644
--- a/activemodel/lib/active_model/attribute_methods.rb
+++ b/activemodel/lib/active_model/attribute_methods.rb
@@ -109,7 +109,7 @@ def define_attr_method(name, value=nil, &block)
           # use eval instead of a block to work around a memory leak in dev
           # mode in fcgi
           sing.class_eval <<-eorb, __FILE__, __LINE__ + 1
-            def #{name}; #{value.to_s.inspect}; end
+            def #{name}; #{value.nil? ? 'nil' : value.to_s.inspect}; end
           eorb
         end
       end
diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 5ae1ebc9cf7dd1712ee1a82ea4a0c069348543b6..c016e936cb59d779f1e97398ed866e33cfad8033 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -24,7 +24,7 @@ def reset_primary_key #:nodoc:
         end
 
         def get_primary_key(base_name) #:nodoc:
-          return unless base_name
+          return unless base_name && !base_name.blank?
 
           case primary_key_prefix_type
           when :table_name
@@ -32,7 +32,11 @@ def get_primary_key(base_name) #:nodoc:
           when :table_name_with_underscore
             base_name.foreign_key
           else
-            'id'
+            if ActiveRecord::Base != self && connection.table_exists?(table_name)
+              connection.primary_key(table_name)
+            else
+              'id'
+            end
           end
         end
 
diff --git a/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb b/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
index 234696adeb2b896749dd51de3b6b1cc04baba56f..b8abdface441acf8b1853db603a5d21931ebe287 100644
--- a/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
+++ b/activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb
@@ -9,6 +9,11 @@ def setup
                                        :timeout => 100
       end
 
+      def test_primary_key_returns_nil_for_no_pk
+        @conn.exec_query('create table ex(id int, data string)')
+        assert_nil @conn.primary_key('ex')
+      end
+
       def test_connection_no_db
         assert_raises(ArgumentError) do
           Base.sqlite3_connection {}
diff --git a/activerecord/test/cases/associations/join_model_test.rb b/activerecord/test/cases/associations/join_model_test.rb
index 0a57c7883fcb39b43abda040f0d9b309214ce0cc..58542bc9397179721ff11d5a87c659f1580fdb64 100644
--- a/activerecord/test/cases/associations/join_model_test.rb
+++ b/activerecord/test/cases/associations/join_model_test.rb
@@ -512,6 +512,10 @@ def test_create_associate_when_adding_to_has_many_through
     assert_nothing_raised { vertices(:vertex_1).sinks << vertices(:vertex_5) }
   end
 
+  def test_add_to_join_table_with_no_id
+    assert_nothing_raised { vertices(:vertex_1).sinks << vertices(:vertex_5) }
+  end
+
   def test_has_many_through_collection_size_doesnt_load_target_if_not_loaded
     author = authors(:david)
     assert_equal 10, author.comments.size
diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index dc5e91241244a8c9bc89acaf42f9deb77b7d649b..e186b2aea7890e2abc0cc3a1bf1d483f74b46012 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -19,6 +19,7 @@
 require 'models/warehouse_thing'
 require 'models/parrot'
 require 'models/loose_person'
+require 'models/edge'
 require 'rexml/document'
 require 'active_support/core_ext/exception'
 
@@ -48,6 +49,10 @@ class Boolean < ActiveRecord::Base; end
 class BasicsTest < ActiveRecord::TestCase
   fixtures :topics, :companies, :developers, :projects, :computers, :accounts, :minimalistics, 'warehouse-things', :authors, :categorizations, :categories, :posts
 
+  def test_primary_key_with_no_id
+    assert_nil Edge.primary_key
+  end
+
   def test_select_symbol
     topic_ids = Topic.select(:id).map(&:id).sort
     assert_equal Topic.find(:all).map(&:id).sort, topic_ids