code: function ensureIndex(coll, fieldOrSpec, options, callback) {
    ensureIndexDb(coll.s.db, coll.s.name, fieldOrSpec, options, callback);
  }
docs: Ensure that an index exists. If the index does not exist, this function creates it.
  
  @method
  @param {Collection} a Collection instance.
  @param {(string|object)} fieldOrSpec Defines the index.
  @param {object} [options] Optional settings. See Collection.prototype.ensureIndex for a list of options.
  @param {Collection~resultCallback} [callback] The command result callback