Mutations in Apollo Kotlin

Mutations are GraphQL operations that modify back-end data. As a convention in GraphQL, queries are read operations and mutations are write operations.

Defining a mutation

You define a mutation in your app just like you define a query, except you use the mutation keyword. Here's an example mutation for upvoting a post:

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mutation UpvotePost($postId: Int!) {
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  upvotePost(postId: $postId) {
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    id
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    votes
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  }
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}

And here's an example schema snippet that supports this mutation:

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type Mutation {
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  upvotePost(postId: Int!): Post
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}
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type Post {
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  id: Int!
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  votes: Int!
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  content: String!
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}

The fields of the Mutation type (such as upvotePost above) usually describe the actions that mutations can perform. These fields usually take one or more arguments, which specify the data to create or modify.

Mutation return types

The return type of a Mutation field usually includes the backend data that's been modified. This provides the requesting client with immediate information about the result of the mutation.

In the example above, upvotePost returns the Post object that's just been upvoted. Here's an example response:

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{
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  "data": {
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    "upvotePost": {
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      "id": "123",
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      "votes": 5
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    }
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  }
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}

For more information on mutation return types, see Designing mutations.

Generating mutation classes

Similar to queries, mutations are represented by instances of generated classes, conforming to the Mutation interface. Constructor arguments are used to define mutation variables. You pass a mutation object to ApolloClient#mutation(mutation) to send the mutation to the server, execute it, and receive typed results:

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val upvotePostMutation = UpvotePostMutation(postId = 3)
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val response = apolloClient.mutation(upvotePostMutation).execute()

Using fragments in mutation results

In many cases, you'll want to use mutation results to update your UI. Fragments are a great way to share result handling between queries and mutations:

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mutation UpvotePost($postId: Int!) {
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  upvotePost(postId: $postId) {
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    ...PostDetails
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  }
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}

Passing input objects

The GraphQL type system includes input objects as a way to pass complex values to fields. Input objects are often used for mutation variables, because they provide a compact way to pass in objects to be created:

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mutation CreateReviewForEpisode($episode: Episode!, $review: ReviewInput!) {
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  createReview(episode: $episode, review: $review) {
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    stars
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    commentary
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  }
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}

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val reviewInput = ReviewInput(stars = 5, commentary = "This is a great movie!")
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val response = apolloClient.mutation(CreateReviewForEpisodeMutation(episode = Episode.NEWHOPE, review = reviewInput)).execute()