fix(discriminator): avoid removing nested path if both base and discr…

…iminator schema have the same nested path

Fix #9362

lib/helpers/model/discriminator.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const Mixed = require('../../schema/mixed');
 const defineKey = require('../document/compile').defineKey;
 const get = require('../get');
 const utils = require('../../utils');
@@ -89,11 +90,12 @@ module.exports = function discriminator(model, name, schema, tiedValue, applyPlu
       if (path.indexOf('.') === -1) {
         continue;
       }
-      const sp = path.split('.');
+      const sp = path.split('.').slice(0, -1);
       let cur = '';
       for (const piece of sp) {
         cur += (cur.length ? '.' : '') + piece;
-        if (schema.paths[cur] || schema.singleNestedPaths[cur]) {
+        if (schema.paths[cur] instanceof Mixed ||
+            schema.singleNestedPaths[cur] instanceof Mixed) {
           conflictingPaths.push(path);
         }
       }