Querying with variables

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Querying with variables

If you’re jumping in here, git checkout 2_1.0.0 (tag 2_1.0.0). Tag 3_1.0.0 contains all the code written in this section.

Let’s make a second query—this time with a variable. If we want to display the sections in a chapter, the simplest way would be to select sections in the ChapterList query:

export default {
  name: 'TableOfContents',
  setup() {
    const { result, loading, error } = useQuery(gql`
      query ChapterList {
        chapters {
          id
          number
          title
          sections {
            id
            number
            title
          }
        }
      }
    `)

Then when a chapter is selected, we display a list of chapter.sections. We might not want to do it this way, however, if the query would return too much data. We also might not be able to, if the schema didn’t have a Chapter.sections field.

In this section, we’ll use a separate query, just so that we can see what it would look like. Let’s create a <SectionList> component that takes the chapter id as a prop and queries for that chapter’s sections:

src/components/SectionList.vue

<template>
  ...
</template>

<script>
import { useQuery } from '@vue/apollo-composable'
import { gql } from '@apollo/client/core'

export default {
  name: 'SectionList',
  props: {
    id: {
      type: Number,
      required: true
    }
  },
  setup(props) {
    const { result, loading, error } = useQuery(
      gql`
        query SectionList($id: Int!) {
          chapter(id: $id) {
            sections {
              id
              number
              title
            }
          }
        }
      `,
      props
    )

    ...
  }
}
</script>

The second argument to useQuery() is the variables. The format of our variables object ({ id: '[chapter-id]' }) matches our props object, so we just pass that. And since the props object is reactive, when a new id prop is passed, Apollo will notice the change and send a new query to the server (or read from the cache when the result is available, since the default Apollo Client network policy is cache-first).

If our props didn’t happen to line up with the variables format—for instance, if we had multiple props, or the prop name was something more descriptive like chapterId—then we could use another reactive variable (like a ref or a reactive object) or a function, like this:

export default {
  name: 'SectionList',
  props: {
    chapterId: {
      type: Number,
      required: true
    }
  },
  setup(props) {
    const { result, loading, error } = useQuery(
      gql`
        query SectionList($id: Int!) {
          chapter(id: $id) {
            sections {
              id
              number
              title
            }
          }
        }
      `,
      () => ({
        id: props.chapterId
      })
    )

We can also pass a non-reactive variables object and then change it later with the variables ref that useQuery() returns:

setup(props) {
  const { result, variables } = useQuery(
    gql`
      query SectionList($id: Int!) {
        chapter(id: $id) {
          sections {
            id
            number
            title
          }
        }
      }
    `,
    { id: 1 }
  )

  function selectChapter (id) {
    variables.value = {
      id,
    }
  }

Going back to our original setup function, after we get the result, we want to extract the sections:

import { useResult } from '@vue/apollo-composable'
import { computed } from 'vue'

export default {
  name: 'SectionList',
  props: {
    id: {
      type: Number,
      required: true
    }
  },
  setup(props) {
    const { result, loading, error } = useQuery(
      gql`
        query SectionList($id: Int!) {
          chapter(id: $id) {
            sections {
              id
              number
              title
            }
          }
        }
      `,
      props
    )

    const sections = useResult(result, [], data => data.chapter.sections)

    return {
      loading,
      error,
      sections,
      noSections: computed(() => sections.value.length === 1)
    }
  }
}

In TableOfContents.vue, we only passed two arguments to useResult(), relying on the default behavior of extracting the root query field. However, the root query field here is chapter, which we don’t want. So we pass a third argument: a function that tells useResult() how to extract the data. Note that we don’t need to guard against data or data.chapter being null, as useResult() catches errors from this function.

When the API returns a single section in a chapter, it means the chapter consists of a single unnamed section, so we can’t display a section list. It will be helpful in the template to have a noSections boolean. We create it using computed() so that it’s reactive, updating when the sections ref changes (which in turn changes when the props change and useQuery() returns a different result).

Now we’re ready to use our reactive variables loading, error, sections, and noSections in the template:

<template>
  <h2>Sections</h2>

  <div v-if="loading">Loading...</div>

  <div v-else-if="error">Error: {{ error.message }}</div>

  <div v-else-if="noSections">This chapter has no sections</div>

  <ul v-else>
    <li v-for="section of sections" :key="section.id">
      {{ section.number }}. {{ section.title }}
    </li>
  </ul>
</template>

<script>
...
</script>

As before, we use a v-if to cover all the different states and v-for to go through the section list.

Lastly, we need to include SectionList in TableOfContents and pass the id prop like <SectionList id="1">. But we want the id to change when we click a different chapter, so we pass a ref like <SectionList :id="currentSection">. Here’s the full implementation, with an updateCurrentSection() function for updating the ref:

src/components/TableOfContents.vue

<template>
  <div v-if="loading">Loading...</div>

  <div v-else-if="error">Error: {{ error.message }}</div>

  <ul>
    <li v-for="chapter of chapters" :key="chapter.id">
      <a @click="updateCurrentSection(chapter.id)">
        {{
          chapter.number ? `${chapter.number}. ${chapter.title}` : chapter.title
        }}
      </a>
    </li>
  </ul>

  <SectionList :id="currentSection" />
</template>

<script>
import { useQuery, useResult } from '@vue/apollo-composable'
import { gql } from '@apollo/client/core'
import { ref } from 'vue'

import SectionList from './SectionList.vue'

const PREFACE_ID = -2

export default {
  name: 'TableOfContents',
  components: {
    SectionList
  },
  setup() {
    const currentSection = ref(PREFACE_ID)

    const { result, loading, error } = useQuery(gql`
      query ChapterList {
        chapters {
          id
          number
          title
        }
      }
    `)

    const chapters = useResult(result, [])

    return {
      loading,
      error,
      chapters,
      currentSection,
      updateCurrentSection: newSection => (currentSection.value = newSection)
    }
  }
}
</script>

We call updateCurrentSection() when a chapter title is clicked. We can test it out by clicking different chapter titles in the browser and see that it works! 💃

When we click Chapter 6: React, that chapter’s sections appear below.

We can also notice that when we click on a new chapter, we see “Loading…” flash before the sections appear, and when we click on a chapter we’ve clicked on previously, the sections appear immediately, as Apollo Client loaded the data from the cache instead of querying the server.