Deprecate extract! calling private methods

Fix #140

lib/jbuilder.rb

@@ -252,9 +252,9 @@ def array!(collection, *attributes, &block)
   #   json.(@person, :name, :age)
   def extract!(object, *attributes)
     if ::Hash === object
-      attributes.each { |attribute| _set_value attribute, object.fetch(attribute) }
+      _extract_hash_values(object, *attributes)
     else
-      attributes.each { |attribute| _set_value attribute, object.send(attribute) }
+      _extract_method_values(object, *attributes)
     end
   end
 
@@ -285,6 +285,26 @@ def target!
 
   private
 
+    def  _extract_hash_values(object, *attributes)
+      attributes.each{ |key| _set_value key, object.fetch(key) }
+    end
+
+    def _extract_method_values(object, *attributes)
+      attributes.each do |method_name|
+        value = if object.respond_to?(method_name)
+          object.public_send(method_name)
+        else
+          message = "Private method #{method_name.inspect} was used to " +
+            'extract value. This will be an error in future versions of Jbuilder'
+
+          ::ActiveSupport::Deprecation.warn message
+          object.send(method_name)
+        end
+
+        _set_value method_name, value
+      end
+    end
+
     def _set_value(key, value)
       raise NullError, key if @attributes.nil?
       unless @ignore_nil && value.nil?

test/jbuilder_test.rb

@@ -73,6 +73,23 @@ class JbuilderTest < ActiveSupport::TestCase
     assert_equal 32, parsed['age']
   end
 
+  test 'extracting from object using private method' do
+    person = Struct.new(:name) do
+      private
+      def age
+        32
+      end
+    end.new('David')
+
+    message = 'Private method :age was used to extract value. This will be' +
+      ' an error in future versions of Jbuilder'
+
+    ::ActiveSupport::Deprecation.expects(:warn).with(message)
+    json = Jbuilder.encode do |json|
+      json.extract! person, :name, :age
+    end
+  end
+
   test 'extracting from object using call style for 1.9' do
     person = Struct.new(:name, :age).new('David', 32)