Common behavior with adding formats to lookup_context for TemplateRenderer and PartialRenderer

actionpack/lib/action_view/lookup_context.rb

@@ -103,7 +103,7 @@ module ActionView
 
     # Helpers related to template lookup using the lookup context information.
     module ViewPaths
-      attr_reader :view_paths
+      attr_reader :view_paths, :html_fallback_for_js
 
       # Whenever setting view paths, makes a copy so we can manipulate then in
       # instance objects as we wish.
@@ -200,7 +200,10 @@ module ActionView
     def formats=(values)
       if values
         values.concat(default_formats) if values.delete "*/*"
-        values << :html if values == [:js]
+        if values == [:js]
+          values << :html
+          @html_fallback_for_js = true
+        end
       end
       super(values)
     end

actionpack/lib/action_view/renderer/abstract_renderer.rb

@@ -37,5 +37,11 @@ module ActionView
     def instrument(name, options={})
       ActiveSupport::Notifications.instrument("render_#{name}.action_view", options){ yield }
     end
+
+    def prepend_formats(formats)
+      formats = Array(formats)
+      return if formats.empty? || @lookup_context.html_fallback_for_js
+      @lookup_context.formats = formats | @lookup_context.formats
+    end
   end
 end

actionpack/lib/action_view/renderer/partial_renderer.rb

@@ -281,6 +281,8 @@ module ActionView
       @block   = block
       @details = extract_details(options)
 
+      prepend_formats(options[:formats])
+
       if String === partial
         @object     = options[:object]
         @path       = partial

actionpack/lib/action_view/renderer/template_renderer.rb

@@ -10,9 +10,10 @@ module ActionView
       template = determine_template(options)
       context  = @lookup_context
 
+      prepend_formats(template.formats)
+
       unless context.rendered_format
-        context.formats = template.formats unless template.formats.empty?
+        context.rendered_format = template.formats.first || formats.last
-        context.rendered_format = context.formats.first
       end
 
       render_template(template, options[:layout], options[:locals])

actionpack/test/fixtures/test/_changing_priority.html.erb

@@ -0,0 +1 @@
+HTML

actionpack/test/fixtures/test/_changing_priority.json.erb

@@ -0,0 +1 @@
+JSON

actionpack/test/fixtures/test/_first_json_partial.json.erb

@@ -0,0 +1 @@
+<%= render :partial => "test/second_json_partial" %>

actionpack/test/fixtures/test/_second_json_partial.json.erb

@@ -0,0 +1 @@
+Third level

actionpack/test/fixtures/test/change_priorty.html.erb

@@ -0,0 +1,2 @@
+<%= render :partial => "test/json_change_priority", formats: :json %>
+HTML Template, but <%= render :partial => "test/changing_priority" %> partial

actionpack/test/fixtures/test/html_template.html.erb

@@ -0,0 +1 @@
+<%= render :partial => "test/first_json_partial", formats: :json %>

actionpack/test/template/render_test.rb

@@ -54,6 +54,16 @@ module RenderTestCases
     assert_equal "Hello world", @view.render(:template => "test/one", :formats => [:html])
   end
 
+  def test_render_partial_implicitly_use_format_of_the_rendered_partial
+    @view.lookup_context.formats = [:html]
+    assert_equal "Third level", @view.render(:template => "test/html_template")
+  end
+
+  def test_render_partial_use_last_prepended_format_for_partials_with_the_same_names
+    @view.lookup_context.formats = [:html]
+    assert_equal "\nHTML Template, but JSON partial", @view.render(:template => "test/change_priorty")
+  end
+
   def test_render_template_with_a_missing_partial_of_another_format
     @view.lookup_context.formats = [:html]
     assert_raise ActionView::Template::Error, "Missing partial /missing with {:locale=>[:en], :formats=>[:json], :handlers=>[:erb, :builder]}" do