code: function FieldReference(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 FieldReference.
  @memberof google.firestore.v1.StructuredQuery
  @interface IFieldReference
  @property {string|null} [fieldPath] FieldReference fieldPath
   
  Constructs a new FieldReference.
  @memberof google.firestore.v1.StructuredQuery
  @classdesc Represents a FieldReference.
  @implements IFieldReference
  @constructor
  @param {google.firestore.v1.StructuredQuery.IFieldReference=} [properties] Properties to set