code: function UnaryFilter(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 an UnaryFilter.
  @memberof google.firestore.v1.StructuredQuery
  @interface IUnaryFilter
  @property {google.firestore.v1.StructuredQuery.UnaryFilter.Operator|null} [op] UnaryFilter op
  @property {google.firestore.v1.StructuredQuery.IFieldReference|null} [field] UnaryFilter field
   
  Constructs a new UnaryFilter.
  @memberof google.firestore.v1.StructuredQuery
  @classdesc Represents an UnaryFilter.
  @implements IUnaryFilter
  @constructor
  @param {google.firestore.v1.StructuredQuery.IUnaryFilter=} [properties] Properties to set