code: function Filter(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 Filter.
  @memberof google.firestore.v1.StructuredQuery
  @interface IFilter
  @property {google.firestore.v1.StructuredQuery.ICompositeFilter|null} [compositeFilter] Filter compositeFilter
  @property {google.firestore.v1.StructuredQuery.IFieldFilter|null} [fieldFilter] Filter fieldFilter
  @property {google.firestore.v1.StructuredQuery.IUnaryFilter|null} [unaryFilter] Filter unaryFilter
   
  Constructs a new Filter.
  @memberof google.firestore.v1.StructuredQuery
  @classdesc Represents a Filter.
  @implements IFilter
  @constructor
  @param {google.firestore.v1.StructuredQuery.IFilter=} [properties] Properties to set