fix(discriminator): don't treat $meta as defining projection when querying

Fix #5859

Author: vkarpov15

lib/queryhelpers.js

@@ -4,6 +4,7 @@
  */
 
 var get = require('lodash.get');
+var isDefiningProjection = require('./services/projection/isDefiningProjection');
 var utils = require('./utils');
 
 /*!
@@ -90,7 +91,8 @@ exports.applyPaths = function applyPaths(fields, schema) {
         continue;
       }
       field = fields[keys[ki]];
-      if (typeof field === 'object' && field && field.$slice) {
+      // Skip `$meta` and `$slice`
+      if (!isDefiningProjection(field)) {
         continue;
       }
       exclude = field === 0;