Support for nested .select() calls (#2737)

* Reproduction for #1567

* Recursive handling of nested pointer keys in select

* Better support for multi-level nested keys

* Adds support for selecting columns natively (mongo)

* Support for postgres column selections

* Filter-out empty keys for pg

src/Adapters/Storage/Mongo/MongoTransform.js

@@ -11,9 +11,11 @@ const transformKey = (className, fieldName, schema) => {
     case 'updatedAt': return '_updated_at';
     case 'sessionToken': return '_session_token';
   }
-
+  
   if (schema.fields[fieldName] && schema.fields[fieldName].__type == 'Pointer') {
     fieldName = '_p_' + fieldName;
+  } else if (schema.fields[fieldName] && schema.fields[fieldName].type == 'Pointer') {
+    fieldName = '_p_' + fieldName;
   }
 
   return fieldName;