import { Types } from 'mongoose';
type ObjectId = Types.ObjectId;
export type Ref<T> = string | ObjectId | (T & {
    _id: string | ObjectId;
});
export type Nullable<T> = T | null | undefined;
export declare const isRef: <T>(obj: unknown) => obj is Ref<T>;
export declare const isPopulated: <T>(ref: Ref<T>) => ref is T & {
    _id: string | ObjectId;
};
export declare const getIdForRef: <T>(ref: Ref<T>) => string | ObjectId;
export declare const getIdStringForRef: <T>(ref: Ref<T>) => string;
export {};