do not use any Entity constant that is available in namespace, as it can...

CHANGELOG.md

@@ -21,6 +21,7 @@ Next Release
 
 #### Fixes
 
+* [#600](https://github.com/intridea/grape/pull/600): Do not use any Entity constant that is available in namespace as presenter - [@fuksito](https://github.com/fuksito)
 * [#590](https://github.com/intridea/grape/pull/590): Fix Issue Where Endpoint Param of Type Integer Cannot Set Values Array - [@xevix](https://github.com/xevix)
 * [#586](https://github.com/intridea/grape/pull/586): Do not repeat the same validation error messages - [@kiela](https://github.com/kiela)
 * [#508](https://github.com/intridea/grape/pull/508): Allow parameters, such as content encoding, in `content_type` - [@dm1try](https://github.com/dm1try).

lib/grape/endpoint.rb

@@ -332,7 +332,7 @@ def present(*args)
           entity_class ||= (settings[:representations] || {})[potential]
         end
 
-        entity_class ||= object_instance.class.const_get(:Entity) if object_instance.class.const_defined?(:Entity)
+        entity_class ||= object_instance.class.const_get(:Entity) if object_instance.class.const_defined?(:Entity) && object_instance.class.const_get(:Entity).respond_to?(:represent)
       end
 
       root = options.delete(:root)

spec/grape/entity_spec.rb

@@ -121,6 +121,19 @@ def first
       get '/example'
     end
 
+    it 'autodetection does not use Entity if it is not a presenter' do
+      some_model = Class.new
+      entity = Class.new
+
+      some_model.class.const_set :Entity, entity
+
+      subject.get '/example' do
+        present some_model
+      end
+      get '/example'
+      entity.should_not_receive(:represent)
+    end
+
     it 'adds a root key to the output if one is given' do
       subject.get '/example' do
         present({ abc: 'def' }, root: :root)