fix(db): one possible fix for Automattic/mongoose#3864

Author: vkarpov15

lib/db.js

@@ -21,6 +21,8 @@ var EventEmitter = require('events').EventEmitter
   , Collection = require('./collection')
   , crypto = require('crypto');
 
+var INVALID_PARTIAL_INDEX_EXPRESSION_CODE = 67;
+
 var debugFields = ['authSource', 'w', 'wtimeout', 'j', 'native_parser', 'forceServerObjectId'
   , 'serializeFunctions', 'raw', 'promoteLongs', 'bufferMaxEntries', 'numberOfRetries', 'retryMiliSeconds'
   , 'readPreference', 'pkFactory'];
@@ -994,6 +996,9 @@ var createIndex = function(self, name, fieldOrSpec, options, callback) {
   // Attempt to run using createIndexes command
   createIndexUsingCreateIndexes(self, name, fieldOrSpec, options, function(err, result) {
     if(err == null) return handleCallback(callback, err, result);
+    if(err.code === INVALID_PARTIAL_INDEX_EXPRESSION_CODE) {
+      return handleCallback(callback, err, result);
+    }
     // Create command
     var doc = createCreateIndexCommand(self, name, fieldOrSpec, options);
     // Set no key checking