We used queries in GraphQL for reading our data. If we want to write data or modify data from the client we use mutations.
Much like the Query type, the Mutation type serves as an entry point to our schema. It follows the same syntax as the schema definition language, or SDL. We declare the Mutation type using the type keyword, then the name Mutation.
Inside the curly braces, we have our entry points, the fields we'll be using to mutate our data.
When we use a mutation to modify more than object we return all of the objects that are modified to show that the mutation has been successful.
Mutations are defined just like queries are. We will add a mutation resolver inside the mutation object which also needs to have the same name as in our schema. We can put the mutation in a try-catch block to easily handle any errors that we might face.
We can also be more dynamic and use the values that Apollo Server and the RESTDataSource class provide. When an error occurs, Apollo Server attaches an extensions field to that error that contains relevant error details.
We start with the mutation keyword, and then the name of the operation (Adding a verb like add or delete is really helpful when creating mutations.) You can use variables to filter when updating specific data.
When we want to execute the mutation from our front-end application we can use the useMutation hook that Apollo provides us. Unlike with useQuery, calling useMutation doesn't actually execute the mutation automatically. Instead, the useMutation hook returns an array with two elements.
- The first element is the mutate function we'll use to actually run the mutation later on.
- The second element is an object with information about the mutation:
loading,erroranddata.
You can use an onClick method or any other way you want to trigger the mutation when you want to update data from a GraphQL API.