Data builders

Apollo Kotlin generates models for your operations and parsers that create instances of these models from your network responses. Sometimes though, during tests or in other occasions, it is useful to instantiate models manually with known values.

Doing so is not as straightforward as it may seem, especially when fragments are used. Creating operationBased models requires instantiating every fragment as well as choosing an appropriate __typename for each composite type.

Data builders make this easier by providing builders that match the structure of the Json document.

Enabling data builders

To enable data builders, set the generateDataBuilders option to true:

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apollo {
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  service("service") {
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    // ...
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    // Enable data builder generation
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    generateDataBuilders.set(true)
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  }
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}

This generates a builder for each composite type in your schema as well as a helper Data {} function for each of your operations.

Example usage

Let's say we're building a test that uses a mocked result of the following query:

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query HeroForEpisode($ep: Episode!) {
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  hero(episode: $ep) {
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    firstName
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    lastName
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    age
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    ship {
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      model
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      speed
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    }
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    friends {
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      firstName
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      lastName
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    }
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    ... on Droid {
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      primaryFunction
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    }
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    ... on Human {
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      height
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    }
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  }
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}

Here's how we can use the corresponding data builder for that mocked result:

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@Test
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fun test() {
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  val data = HeroForEpisodeQuery.Data {
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    // Set values for particular fields of the query
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    hero = buildHuman {
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      firstName = "John"
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      age = 42
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      friends = listOf(
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        buildHuman {
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          firstName = "Jane"
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        }, 
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        buildHuman {
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          lastName = "Doe"
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        }
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      )
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      ship = buildStarship {
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        model = "X-Wing"
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      }
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    }
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  }
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  assertEquals("John", data.hero.firstName)
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  assertEquals(42, data.hero.age)
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}

In this example, the hero field is a Human object with specified values for firstName and age. The values for lastNameand height are automatically populated with mock values. Similarly, values for the ship's speed, the 1st friend's last name and 2nd friend's first name are automatically populated.

Aliases

Because data builders are schema-based and aliases are defined in your queries, there is no way for the codegen to generate builder fields for them. Instead, you'll need to specify them explicitly.

Given a query like this:

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query GetHeroes {
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  luke: hero(id: "1002") {
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    name
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  }
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  leia: hero(id: "1003") {
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    name
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  }
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}

You can generate a fake data model like so:

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val data = GetHeroes.Data {
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  this["luke"] = buildHumanHero {
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    name = "Luke"
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  }
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  this["leia"] = buildHumanHero {
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    name = "Leia"
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  }
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}

@skip and @include directives

By default, the data builders match the types in your schema. If a field is non-null you will either have to provide a value or let the default resolver provide one. This is an issue for @skip and @include directives where a field might be absent even if it is non-nullable. To account for this case, use the same syntax as for aliases and set the value to Optional.Absent.

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query Skip($skip: Boolean!) {
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  nonNullableInt @skip(if: $skip)
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}

You can generate a fake data model like so:

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val data = SkipQuery.Data {
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  this["nonNullableInt"] = Optional.Absent
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}
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assertNull(data.nonNullableInt)

Configuring default field values

To assign default values to fields, data builders use an implementation of the FakeResolver interface. By default, they use an instance of DefaultFakeResolver.

The DefaultFakeResolver gives each String field the field's name as its default value, and it increments a counter as it assigns default values for Int fields. It defines similar default behavior for other types.

You can create your own FakeResolver implementation (optionally delegating to DefaultFakeResolver for a head start). You then pass this implementation as a parameter to the Data function, like so:

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// A FakeResolver implementation that assigns -1 to all Int fields
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class MyFakeResolver : FakeResolver {
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  private val delegate = DefaultFakeResolver(__Schema.all)
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  override fun resolveLeaf(context: FakeResolverContext): Any {
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    return when (context.mergedField.type.leafType().name) {
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      "Int" -> -1 // Always use -1 for Int
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      else -> delegate.resolveLeaf(context) 
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    }
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  }
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  override fun resolveListSize(context: FakeResolverContext): Int {
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    // Delegate to the default behaviour
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    return delegate.resolveListSize(context)
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  }
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  override fun resolveMaybeNull(context: FakeResolverContext): Boolean {
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    // Never 
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    return false
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  }
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  override fun resolveTypename(context: FakeResolverContext): String {
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    // Delegate to the default behaviour
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    return delegate.resolveTypename(context)
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  }
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}
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@Test
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fun test() {
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  val data = HeroForEpisodeQuery.Data(resolver = MyFakeResolver()) {
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    hero = buildHuman {
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      firstName = "John"
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    }
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  }
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  // Unspecified Int field is -1
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  assertEquals(-1, data.hero.age)
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}