Add log message when loading records from Identity Map.

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

activerecord/lib/active_record/identity_map.rb

@@ -50,7 +50,14 @@ def without
 
       def get(klass, primary_key)
         obj = repository[klass.symbolized_base_class][primary_key]
-        obj.is_a?(klass) ? obj : nil
+        if obj.is_a?(klass)
+          if ActiveRecord::Base.logger
+            ActiveRecord::Base.logger.debug "#{klass} with ID = #{primary_key} loaded from Identity Map"
+          end
+          obj
+        else
+          nil
+        end
       end
 
       def add(record)

activerecord/test/cases/identity_map_test.rb

@@ -388,6 +388,15 @@ def test_find_using_select_and_identity_map
     assert_not_nil post.title
   end
 
+  def test_log
+    log = StringIO.new
+    ActiveRecord::Base.logger = Logger.new(log)
+    ActiveRecord::Base.logger.level = Logger::DEBUG
+    Post.find 1
+    Post.find 1
+    assert_match(/Post with ID = 1 loaded from Identity Map/, log.string)
+  end
+
 # Currently AR is not allowing changing primary key (see Persistence#update)
 # So we ignore it. If this changes, this test needs to be uncommented.
 #  def test_updating_of_pkey