fix(query): don't explicitly project in discriminator key if user pro…

…jected in parent path

Fix #5775

lib/queryhelpers.js

@@ -3,6 +3,7 @@
  * Module dependencies
  */
 
+var get = require('lodash.get');
 var utils = require('./utils');
 
 /*!
@@ -75,7 +76,6 @@ exports.createModel = function createModel(model, doc, fields, userProvidedField
 
 exports.applyPaths = function applyPaths(fields, schema) {
   // determine if query is selecting or excluding fields
-
   var exclude;
   var keys;
   var ki;
@@ -123,8 +123,25 @@ exports.applyPaths = function applyPaths(fields, schema) {
     }
 
     // check for parent exclusions
-    var root = path.split('.')[0];
-    if (~excluded.indexOf(root)) return;
+    var pieces = path.split('.');
+    var root = pieces[0];
+    if (~excluded.indexOf(root)) {
+      return;
+    }
+
+    // Special case: if user has included a parent path of a discriminator key,
+    // don't explicitly project in the discriminator key because that will
+    // project out everything else under the parent path
+    if (!exclude && get(type, 'options.$skipDiscriminatorCheck', false)) {
+      var cur = '';
+      for (var i = 0; i < pieces.length; ++i) {
+        cur += (cur.length === 0 ? '' : '.') + pieces[i];
+        var projection = get(fields, cur, false);
+        if (projection && typeof projection !== 'object') {
+          return;
+        }
+      }
+    }
 
     (type.selected ? selected : excluded).push(path);
   };