Tim Deschryver

Parameterized selectors

How can I select an entity from the store by its id?

This question popped up several times lately and in this post I’ll provide some suggestions to create a selector that works with parameters, so let’s not waste any time and let’s get started!

NgRx 6.1

The following will be more or less a copy of the NgRx docs.

As of NgRx 6.1 selectors also accepts an extra props argument. Which means you can now define a selector as the following:

export const getCount = createSelector(
  getCounterValue,
  (counter, props) => counter * props.multiply,
)

Inside your component you can use the selector as usual but you can define the props value:

this.counter = this.store.pipe(select(fromRoot.getCount, { multiply: 2 }))

Keep in mind that a selector is memoized, meaning it will cache the result from the last parameters. If you would re-use the above selector but with other another props value, it would clear the previous cache. If both selectors would be used at the same time, as in the example below, the selector would be invoked with every props value, thus the memoization would be more or less useless.

this.counter2 = this.store.pipe(
  select(fromRoot.getCount, { id: 'counter2', multiply: 2 }),
)
this.counter4 = this.store.pipe(
  select(fromRoot.getCount, { id: 'counter4', multiply: 4 }),
)

Every time the counter value changes in the above example, the selector would be invoked 2 times, one time for counter2, the other time for counter4. To allow memoization we can use a factory function to create the selector.

export const getCount = () =>
  createSelector(
    (state, props) => state.counter[props.id],
    (counter, props) => counter * props.multiply,
  )

And in our component we can invoke the factory function fromRoot.getCount() to create a new selector instance for each counter, allowing each instance to have its own cache.

this.counter2 = this.store.pipe(
  select(fromRoot.getCount(), { id: 'counter2', multiply: 2 }),
)
this.counter4 = this.store.pipe(
  select(fromRoot.getCount(), { id: 'counter4', multiply: 4 }),
)

If the parameter doesn’t change over time we can use a factory function selectCustomer which has a parameter id and returns a selector. Making it possible to use the id parameter in our selector to retrieve the customer, resulting in the following:

export const selectCustomer = (id: string) =>
  createSelector(selectCustomers, customers => customers[id])

We can then call selectCustomer in the component and pass it an id:

this.customer = store.pipe(select(customers.selectCustomer('47')))

If the id parameter is dynamic we can create a selector that instead of returning customers, returns a function which expects a parameter id. The selector becomes:

export const selectCustomer = createSelector(
  selectCustomers,
  customers => (id: string) => customers[id],
)

// tip: it’s also possible to memoize the function if needed
export const selectCustomer = createSelector(selectCustomers, customers =>
  memoize((id: string) => customers[id]),
)

And in our component:

this.customers = this.store.pipe(select(customers.selectCustomer))

For this example I’m also going to show the html, because it’s maybe not that straight forward. Because the selector returns a function now, we can call it like a normal function in our html:

{{ (customers | async)(id).name }}

To overcome this syntax inside the HTML we can also solve this with the RxJS map operator, as mentioned by Juliano Pável.

this.customer = store.pipe(
  select(customers.selectCurrentCustomer),
  map(fun => fun(this.customerId)),
)

Aaaand that’s it, ship it! Or maybe not that fast… While this example works, to me selecting a part from the store like this feels a bit dirty and I consider it a bad practice in most of the cases. In my opinion the id parameter should exist somewhere in the store.

This way works with a property selectedCustomerId in the store. To select a customer we first have to fire an action SelectCustomer with the customer’s id as payload to set the selectedCustomerId in the store.

this.store.dispatch(new SelectCustomer(customer.id))

We also have to create a selector selectSelectedCustomerId to select the selectedCustomerId. Because selectors are composable we can now use both selectors to get the selected customer in getSelectedCustomer. The selector now looks like this:

export const selectSelectedCustomerId = createSelector(
  selectCustomerEntities,
  fromCustomers.selectedCustomerId,
)

export const getSelectedCustomer = createSelector(
  selectCustomers,
  selectSelectedCustomerId,
  (customers, selectedId) => customers[selectedId],
)

And in the component we can use the selector the ‘usual way’:

this.customer = store.pipe(select(customers.getSelectedCustomer))

Another possibility would be to use @ngrx/router-store, this module connects the route with the store. In other words, all the route information will be available in the store thus also in the selectors. After installing the ngrx/router-store module and having it imported in our AppModule, we’ll first have to create a selector selectRouteParameters to retrieve the route parameters (_customerId_ in our case). Thereafter we can use the created selector in selectCurrentCustomer to select the current customer. This means that when a user clicks on a link or navigates directly to /customers/47, he or she would see the customer’s details of customer 47. The selector looks roughly the same:

export const selectRouterState = createFeatureSelector<RouterReducerState>(
  'router',
)

export const selectRouteParameters = createSelector(
  selectRouterState,
  router => router.state.root.firstChild.params,
)

export const selectCurrentCustomer = createSelector(
  selectCustomers,
  selectRouteParameters,
  (customers, route) => customers[route.customerId],
)

And the component remains the same (except for the selector’s name):

this.customer = store.pipe(select(customers.selectCurrentCustomer))

In my opinion the code we ended up with looks cleaner than the code we started of with. I hope this was useful if you were looking for a way to parameterize your selector.

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