2. What is a mutation?
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🖋 GraphQL operations

So far, our app has only used one type of : queries. These are read-only operations to retrieve data. To modify data, we need to use another type of GraphQL operation: mutations, which are write operations.

Much like the Query type, the Mutation type serves as an entry point to our schema. It follows the same syntax as the , or , that we've been using so far.

✍️ Schema syntax

We declare the Mutation type using the type keyword, then the name Mutation. Inside the curly braces, we have our entry points, the we'll be using to mutate our data.

Illustration showing the schema syntax for adding a mutation

We recommend starting with a verb that describes the specific action of our update (such as add, delete, or create), followed by whatever data the acts on. Because mutations typically modify a specific object, they often require . We can add arguments as needed, following the same syntax. The return type of the mutation comes after the colon.

For the response, we should return the data that the mutation updated, so that our client can update its UI without having to run a followup .

Which of these are good names for mutations based on the recommended conventions above?

🤔 Modifying multiple objects

When a updates more than just one object, how do we pick which one to return?

To answer that question, let's look at an example schema with spacecats and missions.

Illustrating showing the schema syntax for spacecats and missions

A spacecat has a list of missions they've been assigned to. A mission can also have more than one spacecat assigned to it, like a crew. We want to create a called assignMission that assigns one spacecat to a particular mission. This would update that spacecat's list of missions, and it would also update a mission's list of crew members.

So which type should our return? SpaceCat? Or Mission?

We'll want to return both, because our client might need both.

In addition, we'll need to account for any partial errors that might occur and return helpful information to the client. We recommend adding three common to all responses:

  • code: an Int that refers to the status of the response, similar to an HTTP status code.

  • success: a Boolean flag that indicates whether all the updates the was responsible for succeeded.

  • message: a String to display information about the result of the on the client side. This is particularly useful if the mutation was only partially successful and a generic error message can't tell the whole story.

Illustration showing the fields on the mutation type and the fields the response should include
Which of these are reasons to create a separate object for a mutation's return type?

Circling back to the assignMission , we're going to create a new type specifically for its response. By convention, this return type will start with the name of the (AssignMission) and end with Response. This type will contain the three informational properties, as well as additional for each object that the updated.

Illustration showing the new AssignMissionResponse return type
Mutations vs Queries
Queries and mutations are both types of GraphQL
. Queries are 
operations that always 
 data. Mutations are
 operations that always 
data. Similar to Query fields, fields of the 
 type are also 
 into a GraphQL API.

Drag items from this box to the blanks above

  • operations

  • Mutation

  • endpoints

  • read

  • delete

  • modify

  • create

  • retrieve

  • write

  • entry points

  • assignments

That's the basics of what we need to know about ! Let's dive into the code and add our first mutation to the schema.


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