code: function QueryTarget(properties) {
                          if (properties)
                              for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                                  if (properties[keys[i]] != null)
                                      this[keys[i]] = properties[keys[i]];
                      }
docs: Properties of a QueryTarget.
  @memberof google.firestore.v1.Target
  @interface IQueryTarget
  @property {string|null} [parent] QueryTarget parent
  @property {google.firestore.v1.IStructuredQuery|null} [structuredQuery] QueryTarget structuredQuery
   
  Constructs a new QueryTarget.
  @memberof google.firestore.v1.Target
  @classdesc Represents a QueryTarget.
  @implements IQueryTarget
  @constructor
  @param {google.firestore.v1.Target.IQueryTarget=} [properties] Properties to set