constructor() {
    this._nextId = 1;
    this._ids = {};
  }
By default Mirage uses autoincrementing numbers starting with `1` as IDs for records. This can be customized by implementing one or more IdentityManagers for your application.

  An IdentityManager is a class that's responsible for generating unique identifiers. You can define a custom identity manager for your entire application, as well as on a per-model basis.

  A custom IdentityManager must implement these methods:

  - `fetch`, which must return an identifier not yet used
  - `set`, which is called with an `id` of a record being insert into Mirage's database
  - `reset`, which should reset database to initial state

  Check out the advanced guide on Mocking UUIDs to see a complete example of a custom IdentityManager.

  @class IdentityManager
  @constructor
  @public