Mocking GUIDs

Some applications use GUIDs (or UUIDs) instead of auto-incrementing integers as identifiers for their models.

Mirage supports the ability to overwrite how its database assigns IDs to new records via the IdentityManager class. You can generate model-specific managers or an application-wide manager to customize how your database behaves.

A custom identity manager must implement these methods:

  • fetch, which must return an identifier not used yet.
  • set, which is called with an id of a record being insert in mirage's database.
  • reset, which should reset database to initial state.

Here's an example implementation for an identity manager that mocks GUIDs:

import uuidv4 from "uuid/v4"

export default class {
  constructor() {
    this.ids = new Set()

  // Returns a new unused unique identifier.
  fetch() {
    let uuid = uuidv4()
    while (this.ids.has(uuid)) {
      uuid = uuidv4()


    return uuid

  // Registers an identifier as used. Must throw if identifier is already used.
  set(id) {
    if (this.ids.has(id)) {
      throw new Error(`ID ${id} has already been used.`)


  // Resets all used identifiers to unused.
  reset() {