Map a custom identifier field
Entities use a primary-key property named something other than id.
Redux Toolkit · Type Alias
IdSelector
IdSelector<T> describes a function that extracts an entity's unique EntityId from a model. It is used by createEntityAdapter, with entity.id as the default extraction rule.
How it works
IdSelector<T> is the function type used to obtain an entity's unique identifier from a model. Pass one to createEntityAdapter when the identifier is not stored in the conventional id property.
Syntax
export type IdSelector<T> = (model: T) => EntityIdParameters
| Name | Type | Description |
|---|---|---|
model | T | The model whose unique identifier should be extracted. |
Returns
EntityId
Key points
- IdSelector<T> is a generic function type: it receives a model and returns that model's EntityId.
- Its job is to extract the unique identifier from one entity instance.
- createEntityAdapter uses entity.id by default when no custom selector is supplied.
- Entities whose IDs use another property require a custom selectId function.
- The selector should return the value used as the entity's unique key.
Common mistakes
- A custom selector is required when the entity's identifier is stored in a field other than id.
Example: Using a custom IdSelector
import { createEntityAdapter } from '@reduxjs/toolkit'
type User = { userId: string; name: string }
const usersAdapter = createEntityAdapter<User>({
selectId: (user) => user.userId,
})
const state = usersAdapter.getInitialState()
const nextState = usersAdapter.addOne(state, {
userId: 'u1',
name: 'Ada',
})
console.log(usersAdapter.getSelectors().selectById(nextState, 'u1'))The custom selector extracts each user's unique identifier so the adapter can store and retrieve entities by that identifier.
When to use it
Type adapter configuration
A typed entity adapter needs a reusable contract for ID extraction.
Define normalized entity keys
Normalized state needs stable keys for entity lookup and generated selectors.
Interview questions
QWhat is IdSelector<T> and what does it return?
IdSelector<T> is a function accepting one model of type T and returning an EntityId. It supplies the identifier that the entity adapter uses for normalized storage and lookup.
Follow-up: What should you do if the identifier is not named id? Provide selectId, such as selectId: user => user.userId, when creating the adapter.
QWhen do you need to provide a custom selectId function?
The adapter uses the selector to determine each entity's unique ID. The default behavior uses entity.id, so a custom selector is needed for another identifier field.
Follow-up: How does the selected ID affect adapter selectors? It becomes the key used by selectors such as selectById to find an entity.
QWhy does an entity adapter need an IdSelector?
An IdSelector maps a model to an EntityId, allowing createEntityAdapter to normalize entities into IDs and an entity lookup table. The resulting adapter can then generate selectors for that normalized state.
Follow-up: Which generated selector retrieves one entity? selectById accepts the state and an EntityId, returning the matching entity or undefined.
Related
createEntityAdapterselectIdEntityIdselectByIdselectAllgetSelectors