Ember Data models are normal TypeScript classes, but with properties decorated to define how the model represents an API resource and relationships to other resources. The decorators the library supplies "just work" with TypeScript at runtime, but require type annotations to be useful with TypeScript.

For an overview of using Ember's decorators with TypeScript, see our overview.

@attr

The type returned by the @attr decorator is whatever Transform is applied via the invocation.

• If you supply no argument to @attr, the value is passed through without transformation.

• If you supply one of the built-in transforms, you will get back a corresponding type:

  • @attr('string') → string

  • @attr(number) → number,

  • @attr('boolean') → boolean

  • @attr'date') → Date

• If you supply a custom transform, you will get back the type returned by your transform.

So, for example, you might write a class like this:

import Model, { attr } from '@ember-data/object';
import CustomType from '../transforms/custom-transform';
​
export default class User extends Model {
  @attr()
  name?:  string;
​
  @attr('number')
  declare age: number;
​
  @attr('boolean')
  declare isAdmin: boolean;
​
  @attr('custom-transform')
  declare myCustomThing: CustomType;
}

Very important: Even more than with decorators in general, you should be careful when deciding whether to mark a property as optional ? or definitely present (no annotation): Ember Data will default to leaving a property empty if it is not supplied by the API or by a developer when creating it. That is: the default for Ember corresponds to an optional field on the model.

The safest type you can write for an Ember Data model, therefore, leaves every property optional: this is how models actually behave. If you choose to mark properties as definitely present by leaving off the ?, you should take care to guarantee that this is a guarantee your API upholds, and that ever time you create a record from within the app, you uphold those guarantees.

One way to make this safer is to supply a default value using the defaultValue on the options hash for the attribute:

import Model, { attr } from '@ember-data/object';
​
export default class User extends Model {
  @attr()
  declare name?:  string;
​
  @attr('number', { defaultValue: 13 })
  declare age: number;
​
  @attr('boolean', { defaultValue: false })
  declare isAdmin: boolean;
}

@belongsTo

The type returned by the @hasMany decorator depends on whether the relationship is { async: true } (which it is by default).

• If the value is true, the type you should use is DS.PromiseObject<Model>, where Model is the type of the model you are creating a relationship to.

• If the value is false, the type is Model, where Model is the type of the model you are creating a relationship to.

So, for example, you might define a class like this:

import Model, { belongsTo } from '@ember-data/model';
import DS from 'ember-data'; // NOTE: this is a workaround, see discussion below!
import User from './user';
import Site from './site';
​
export default class Post extends Model {
  @belongsTo('user')
  declare user: DS.PromiseObject<User>;
​
  @belongsTo('site', { async: false })
  declare site: Site;
}

These are type-safe to define as always present, that is to leave off the ? optional marker:

• accessing an async relationship will always return a PromiseObject, which itself may or may not ultimately resolve to a value—depending on the API response—but will always be present itself.

• accessing a non-async relationship which is known to be associated but has not been loaded will trigger an error, so all access to the property will be safe if it resolves at all.

Note, however, that this type-safety is not a guarantee of there being no runtime error: you still need to uphold the contract for non-async relationships (that is: loading the data first, or side-loading it with the request) to avoid throwing an error!

@hasMany

The type returned by the @hasMany decorator depends on whether the relationship is { async: true } (which it is by default).

• If the value is true, the type you should use is DS.PromiseManyArray<Model>, where Model is the type of the model you are creating a relationship to.

• If the value is false, the type is EmberArray<Model>, where Model is the type of the model you are creating a relationship to.

So, for example, you might define a class like this:

import Model, { hasMany } from '@ember-data/model';
import EmberArray from '@ember/array';
import DS from 'ember-data'; // NOTE: this is a workaround, see discussion below!
import Comment from './comment';
import User from './user';
​
export default class Thread extends Model {
  @hasMany('comment')
  declare comment: DS.PromiseManyArray<Comment>;
​
  @hasMany('user', { async: false })
  declare participants: EmberArray<User>;
}

The same basic rules about the safety of these lookups as with @belongsTo apply to these types. The difference is just that in @hasMany the resulting types are arrays rather than single objects.

Importing PromiseObject and PromiseManyArray

There is no public import path in the Ember Data Packages API for the PromiseObject and PromiseManyArray types. These types are slowly being disentangled from Ember Data and will eventually be removed. However, until they are, we need a way to refer to them. For now, the best option is to refer to them via the legacy DS import.

In the future, they will become unnecesary, as the types will simply be Promise<Model> and Promise<Array<Model>>.