# useQuery()

Data rendering without the fetch.

Access any [Queryable Schema](https://dataclient.io/rest/api/schema.md#queryable)'s store value; like [Entity](https://dataclient.io/rest/api/Entity.md), [All](https://dataclient.io/rest/api/All.md), [Collection](https://dataclient.io/rest/api/Collection.md), [Query](https://dataclient.io/rest/api/Query.md),
[Union](https://dataclient.io/rest/api/Union.md), and [Scalar](https://dataclient.io/rest/api/Scalar.md). [Lazy](https://dataclient.io/rest/api/Lazy.md) fields also work via their [`.query`](https://dataclient.io/rest/api/Lazy.md#query) accessor.
If the value does not exist, returns `undefined`.

`useQuery()` is reactive to data [mutations](https://dataclient.io/vue/getting-started/mutations.md); rerendering only when necessary. Returns `undefined`
when data is [Invalid](https://dataclient.io/vue/concepts/expiry-policy.md#invalid).

> **Tip**
>
> [Queries](https://dataclient.io/rest/api/Query.md) are a great companion to efficiently render aggregate computations like those that use [groupBy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/groupBy#browser_compatibility),
> [map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map), [reduce](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce), and [filter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter).

## Usage

```ts title="Post"
import { Entity, EntityMixin } from '@data-client/rest';

export class Post extends Entity {
  id = 0;
  author = { id: 0 };
  title = '';
  body = '';
  votes = 0;

  static key = 'Post';

  static schema = {
    author: EntityMixin(
      class User {
        id = 0;
      },
    ),
  };

  get img() {
    return `//loremflickr.com/96/72/kitten,cat?lock=${this.id % 16}`;
  }
}
```

```ts title="PostResource" {15-22}
import { resource } from '@data-client/rest';
import { Post } from './Post';

export { Post };

export const PostResource = resource({
  path: '/posts/:id',
  searchParams: {} as { userId?: string | number } | undefined,
  schema: Post,
}).extend('vote', {
  path: '/posts/:id/vote',
  method: 'POST',
  body: undefined,
  schema: Post,
  getOptimisticResponse(snapshot, { id }) {
    const post = snapshot.get(Post, { id });
    if (!post) throw snapshot.abort;
    return {
      id,
      votes: post.votes + 1,
    };
  },
});
```

```html title="PostItem.vue" {9}
<script setup lang="ts">
  import { useController } from '@data-client/vue';
  import { PostResource, type Post } from './PostResource';

  const props = defineProps<{ post: Post }>();
  const ctrl = useController();

  const handleVote = () => {
    ctrl.fetch(PostResource.vote, { id: props.post.id });
  };
</script>

<template>
  <div>
    <div class="voteBlock">
      <small class="vote">
        <button class="up" @click="handleVote">&nbsp;</button>
        {{ post.votes }}
      </small>
      <img :src="post.img" width="70" height="52" />
    </div>
    <div>
      <h4>{{ post.title }}</h4>
      <p>{{ post.body }}</p>
    </div>
  </div>
</template>
```

```html title="TotalVotes.vue" {13}
<script setup lang="ts">
  import { Query } from '@data-client/rest';
  import { useQuery } from '@data-client/vue';
  import { PostResource } from './PostResource';

  const queryTotalVotes = new Query(
    PostResource.getList.schema,
    posts => posts.reduce((total, post) => total + post.votes, 0),
  );

  const props = defineProps<{ userId: number }>();
  const totalVotes = useQuery(queryTotalVotes, () => ({ userId: props.userId }));
</script>

<template>
  <div style="text-align: center">
    <small>{{ totalVotes }} votes total</small>
  </div>
</template>
```

```html title="PostList.vue"
<script setup lang="ts">
  import { useSuspense } from '@data-client/vue';
  import { PostResource } from './PostResource';
  import PostItem from './PostItem.vue';
  import TotalVotes from './TotalVotes.vue';

  const userId = 2;
  const posts = await useSuspense(PostResource.getList, { userId });
</script>

<template>
  <div>
    <PostItem v-for="post in posts" :key="post.pk()" :post="post" />
    <TotalVotes :userId="userId" />
  </div>
</template>
```

See [truthiness narrowing](https://www.typescriptlang.org/docs/handbook/2/narrowing.html#truthiness-narrowing) for
more information about type handling

## Types

```typescript
function useQuery<S extends Queryable>(
  schema: S,
  ...args: MaybeRefsOrGetters<SchemaArgs<S>>
): ComputedRef<DenormalizeNullable<S> | undefined>;
```

Arguments can be plain values, [refs](https://vuejs.org/api/reactivity-core.html#ref) (including [computed](https://vuejs.org/api/reactivity-core.html#computed)), or getter
functions like `() => ({ id: props.id })`. A plain object like `{ id: props.id }` is read once and won't
follow prop or route changes, so use a getter or `computed` when an argument can change.

The result updates when the arguments change.

### Queryable

[Queryable](https://dataclient.io/rest/api/schema.md#queryable) schemas require an `queryKey()` method that returns something. These include
[Entity](https://dataclient.io/rest/api/Entity.md), [All](https://dataclient.io/rest/api/All.md), [Collection](https://dataclient.io/rest/api/Collection.md), [Query](https://dataclient.io/rest/api/Query.md),
[Union](https://dataclient.io/rest/api/Union.md), and [Scalar](https://dataclient.io/rest/api/Scalar.md). [Lazy](https://dataclient.io/rest/api/Lazy.md) fields produce a Queryable via their [`.query`](https://dataclient.io/rest/api/Lazy.md#query) accessor.

```ts
interface Queryable {
  queryKey(
    args: readonly any[],
    queryKey: (...args: any) => any,
    getEntity: GetEntity,
    getIndex: GetIndex,
    // Must be non-void
  ): {};
}
```

## Examples

### Sorting & Filtering

[Query](https://dataclient.io/rest/api/Query.md) provides programmatic access to the Reactive Data Client store.

```ts title="UserResource"
import { Entity, resource } from '@data-client/rest';

export class User extends Entity {
  id = '';
  name = '';
  isAdmin = false;

  static key = 'User';
}
export const UserResource = resource({
  path: '/users/:id',
  schema: User,
});
```

```html title="UsersPage.vue" {22}
<script setup lang="ts">
  import { Query, All } from '@data-client/rest';
  import { useQuery, useFetch } from '@data-client/vue';
  import { UserResource, User } from './UserResource';

  interface Args {
    asc: boolean;
    isAdmin?: boolean;
  }
  const sortedUsers = new Query(
    new All(User),
    (entries, { asc, isAdmin }: Args = { asc: false }) => {
      let sorted = [...entries].sort((a, b) =>
        a.name.localeCompare(b.name),
      );
      if (isAdmin !== undefined)
        sorted = sorted.filter(user => user.isAdmin === isAdmin);
      if (asc) return sorted;
      return sorted.reverse();
    },
  );

  useFetch(UserResource.getList);
  const users = useQuery(sortedUsers, { asc: true });
</script>

<template>
  <div v-if="!users">No users in cache yet</div>
  <div v-else>
    <div v-for="user in users" :key="user.pk()">{{ user.name }}</div>
  </div>
</template>
```

### Lazy relationships

[Lazy](https://dataclient.io/rest/api/Lazy.md) fields keep raw IDs during parent denormalization. Use [`.query`](https://dataclient.io/rest/api/Lazy.md#query) with `useQuery` to resolve them on demand,
isolating re-renders to only the components that need the related data.

```ts title="Resources"
import { Entity, Lazy, resource } from '@data-client/rest';

export class Building extends Entity {
  id = '';
  name = '';

  static key = 'Building';
}

export class Department extends Entity {
  id = '';
  name = '';
  buildings: string[] = [];

  static schema = {
    buildings: new Lazy([Building]),
  };
  static key = 'Department';
}

export const DepartmentResource = resource({
  path: '/departments/:id',
  schema: Department,
});
```

```html title="BuildingList.vue" {8-11}
<script setup lang="ts">
  import { computed } from 'vue';
  import { useQuery } from '@data-client/vue';
  import { Department } from './Resources';

  const props = defineProps<{ dept: Department }>();

  const buildings = useQuery(
    Department.schema.buildings.query,
    computed(() => props.dept.buildings),
  );
</script>

<template>
  <span v-if="buildings">{{ buildings.map(b => b.name).join(', ') }}</span>
</template>
```

```html title="DepartmentsPage.vue"
<script setup lang="ts">
  import { All } from '@data-client/rest';
  import { useQuery, useFetch } from '@data-client/vue';
  import { DepartmentResource, Department } from './Resources';
  import BuildingList from './BuildingList.vue';

  useFetch(DepartmentResource.getList);
  const departments = useQuery(new All(Department));
</script>

<template>
  <div v-if="!departments">Loading...</div>
  <div v-else>
    <div v-for="dept in departments" :key="dept.pk()">
      <strong>{{ dept.name }}</strong>: <BuildingList :dept="dept" />
    </div>
  </div>
</template>
```