Dependent Queries

In Riverpod, dependent queries are easily achieved by watching one provider inside another.

Basic Dependent Query

If query B depends on data from query A, you can use ref.watch to ensure query B is updated (or enabled) only when query A has data.


import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:fasq_riverpod/fasq_riverpod.dart';
 
final userProvider = queryProvider<User>(
  'user'.toQueryKey(),
  () => fetchUser(),
);
 
final postsProvider = (String userId) => queryProvider<List<Post>>(
  ['posts', userId].toQueryKey(),
  () => fetchPosts(userId),
);
 
// A provider that reactivey manages the dependency
final userPostsProvider = Provider<AsyncValue<List<Post>>>((ref) {
  final userAsync = ref.watch(userProvider);
 
  return userAsync.when(
    data: (user) => ref.watch(postsProvider(user.id)),
    loading: () => const AsyncValue.loading(),
    error: (e, s) => AsyncValue.error(e, s),
  );
});

Conditional Fetching (Enabled)

You can also use the enabled option to prevent a query from running until its dependency is met.


final postsProvider = queryProvider<List<Post>>(
  'posts'.toQueryKey(),
  () {
    // We can use ref.read here because we've ensured it's enabled
    // only when data is present
    final user = ref.read(userProvider).value!;
    return fetchPosts(user.id);
  },
  options: QueryOptions(
    // Re-evaluate when userProvider changes
    enabled: ref.watch(userProvider).hasValue,
  ),
);

Why use this pattern?

By leveraging Riverpod’s ref.watch, you ensure that:

Automatic Cancellation: If the dependency changes (e.g., user logs out), the dependent query is automatically cancelled or disposed.
Reactivity: Query B automatically refetchs if Query A’s data changes in a way that affects Query B’s parameters.
Type Safety: You get full compile-time safety across your dependency chain.

Next Steps

Family Providers - Passing parameters to queries
Riverpod Patterns - Architecture best practices