fix(mongoose): allow setting useCreateIndex option after creating a m…

…odel but before initial connection succeeds

Fix #6890

lib/model.js

@@ -987,10 +987,7 @@ Model.init = function init(callback) {
   const autoIndex = this.schema.options.autoIndex;
   this.$init = new Promise((resolve, reject) => {
     if (autoIndex || (autoIndex == null && this.db.config.autoIndex)) {
-      const options = {
-        _automatic: true,
-        createIndex: this.base.options.useCreateIndex
-      };
+      const options = { _automatic: true };
       this.ensureIndexes(options, function(error) {
         if (error) {
           return reject(error);
@@ -1211,6 +1208,10 @@ Model.createIndexes = function createIndexes(options, callback) {
   return this.ensureIndexes(options, callback);
 };
 
+/*!
+ * ignore
+ */
+
 function _ensureIndexes(model, options, callback) {
   const indexes = model.schema.indexes();
 
@@ -1271,7 +1272,10 @@ function _ensureIndexes(model, options, callback) {
     applyWriteConcern(model.schema, indexOptions);
 
     indexSingleStart(indexFields, options);
-    const methodName = options.createIndex ? 'createIndex' : 'ensureIndex';
+    const useCreateIndex = 'createIndex' in options ?
+      options.createIndex :
+      model.base.options.useCreateIndex;
+    const methodName = useCreateIndex ? 'createIndex' : 'ensureIndex';
     model.collection[methodName](indexFields, indexOptions, utils.tick(function(err, name) {
       indexSingleDone(err, indexFields, indexOptions, name);
       if (err) {