code: func (c *Collection) EnsureIndexKey(key ...string) error {
  	return c.EnsureIndex(Index{Key: key})
  }
docs: // EnsureIndexKey ensures an index with the given key exists, creating it
  // if necessary.
  //
  // This example:
  //
  //     err := collection.EnsureIndexKey("a", "b")
  //
  // Is equivalent to:
  //
  //     err := collection.EnsureIndex(mgo.Index{Key: []string{"a", "b"}})
  //
  // See the EnsureIndex method for more details.