Dependent Queries

Dependent Queries

useQuery dependent Query

Dependent (or serial) queries depend on previous ones to finish before they can execute. To achieve this, it's as easy as using the enabled option to tell a query when it is ready to run:

tsx

// Get the user
const { data: user } = useQuery({
  queryKey: ['user', email],
  queryFn: getUserByEmail,
})

const userId = user?.id

// Then get the user's projects
const {
  status,
  fetchStatus,
  data: projects,
} = useQuery({
  queryKey: ['projects', userId],
  queryFn: getProjectsByUser,
  // The query will not execute until the userId exists
  enabled: !!userId,
})

// Get the user
const { data: user } = useQuery({
  queryKey: ['user', email],
  queryFn: getUserByEmail,
})

const userId = user?.id

// Then get the user's projects
const {
  status,
  fetchStatus,
  data: projects,
} = useQuery({
  queryKey: ['projects', userId],
  queryFn: getProjectsByUser,
  // The query will not execute until the userId exists
  enabled: !!userId,
})

The projects query will start in:

tsx

status: 'pending'
isPending: true
fetchStatus: 'idle'

status: 'pending'
isPending: true
fetchStatus: 'idle'

As soon as the user is available, the projects query will be enabled and will then transition to:

tsx

status: 'pending'
isPending: true
fetchStatus: 'fetching'

status: 'pending'
isPending: true
fetchStatus: 'fetching'

Once we have the projects, it will go to:

tsx

status: 'success'
isPending: false
fetchStatus: 'idle'

status: 'success'
isPending: false
fetchStatus: 'idle'

useQueries dependent Query

Dynamic parallel query - useQueries can depend on a previous query also, here's how to achieve this:

tsx

// Get the users ids
const { data: userIds } = useQuery({
  queryKey: ['users'],
  queryFn: getUsersData,
  select: (users) => users.map((user) => user.id),
})

// Then get the users messages
const usersMessages = useQueries({
  queries: userIds
    ? userIds.map((id) => {
        return {
          queryKey: ['messages', id],
          queryFn: () => getMessagesByUsers(id),
        }
      })
    : [], // if users is undefined, an empty array will be returned
})

// Get the users ids
const { data: userIds } = useQuery({
  queryKey: ['users'],
  queryFn: getUsersData,
  select: (users) => users.map((user) => user.id),
})

// Then get the users messages
const usersMessages = useQueries({
  queries: userIds
    ? userIds.map((id) => {
        return {
          queryKey: ['messages', id],
          queryFn: () => getMessagesByUsers(id),
        }
      })
    : [], // if users is undefined, an empty array will be returned
})

Note that useQueries return an array of query results

Dependent queries by definition constitutes a form of request waterfall, which hurts performance. If we pretend both queries take the same amount of time, doing them serially instead of in parallel always takes twice as much time, which is especially hurtful when it happens on a client that has high latency. If you can, it's always better to restructure the backend APIs so that both queries can be fetched in parallel, though that might not always be practically feasible.

In the example above, instead of first fetching getUserByEmail to be able to getProjectsByUser, introducing a new getProjectsByUserEmail query would flatten the waterfall.

useQuery dependent Query

useQueries dependent Query

A note about performance

Want to Skip the Docs?

Want to Skip the Docs?