message: Fix definition for Ref<T>
diff: @@ -4,6 +4,7 @@ import * as _ from 'lodash';
  import { schema, virtuals } from './data';
  import { isPrimitive, initAsObject, initAsArray, isString, isNumber, isObject } from './utils';
  import { InvalidPropError, NotNumberTypeError, NotStringTypeError, NoMetadataError } from './errors';
  +import { ObjectID } from 'bson';
  export type Func = (...args: any[]) => any;
  @@ -194,4 +195,4 @@ export const arrayProp = (options: ArrayPropOptions) => (target: any, key: strin
  baseProp(options, Type, target, key, true);
  };
  -export type Ref<T> = T | string;
  +export type Ref<T> = T | ObjectID;
  