Mutations on a GraphQL server built with Vapor

One characteristic of a complete API is its ability to allow clients to modify the server-side data. This can be in the form of creating new entries, updating or deleting existing ones, etc. Building a GraphQL server couldn’t be any different.

To describe this kind of features, GraphQL uses the term Mutations . Mutations are just like normal queries. The only difference is that they make it explicit that they will result in some kind of side-effect on the server-side data.

And that’s where this post will focus on. I am going to describe how to add the mutations to create, edit, and delete an entry on a GraphQL server built with Vapor.

Prerequisites

The code in this post is a continuation of the code from my two latest posts. In the first one, I described how to setup a GraphQL server with Vapor and in the second I focused on how to use custom types on this GraphQL server .

If you want to go through the code as you read the post, the code from the previous posts as well as the code from this post is available on GitHub .

Now, let’s get started!

Implementation

Prepare the models

First, and before we jump into the mutations, we need to adjust our existing models to support the mutations.

For example, when it comes to the edit mutation, we will have to update some properties of the Post model from constant to variable properties.

struct Post: Codable {
     let id: CustomUUID
-    let title: String
+    var title: String
     let publishedAt: Date
-    let tags: [Tag]
+    var tags: [Tag]
     let author: Author
 }

Similarly, in order to be able to filter the posts by the id property, we will have to make CustomUUID conform to the Equatable protocol.

-struct CustomUUID: Codable {
+struct CustomUUID: Codable, Equatable {

With our models ready, we can now move on and start adding the logic for the mutations!

Delete mutation

Let’s start with the logic for the deletePost mutation. For this mutation, we will expect the id of the Post to delete as an argument. Then, using this id , we will search for the Post in the in-memory list. If we manage to find the post, we are going to remove it and return true to the client. Otherwise, we will return false to let the client know that we didn’t manage to find the Post .

For the sake of this post, I am using a Bool as a return type. Ideally, we should return an error Type, but to keep this post focused on the mutations, I decided to go with the Bool .

So, let’s add an extension to the PostController with this logic.

extension PostController {
    struct DeletePostArguments: Codable {
        let id: CustomUUID
    }

    func deletePost(request: Request, arguments: DeletePostArguments) -> Bool {
        let postIndex = posts.firstIndex{ $0.id == arguments.id }
        guard let index = postIndex?.indexValue else {
            return false
        }
        posts.remove(at: index)
        return true
    }
}

Similarly, let’s add the logic for the editPost mutation!

Edit mutation

The edit mutation will allow the user to change the value of the title and the tags for a given post. As a result, this time, we are going to require three arguments; an id , which we will use to find the Post to edit, as well as a title and a tags argument, which will contain the updated values for the title and tags properties respectively.

To keep the logic visually separated, let’s add a new extension to PostController for the logic related to the edit mutation. This extension will contain a new structure named EditPostArguments and a function editPost which will be responsible for editing a post.

The implementation will look like the following snippet:

extension PostController {
    struct EditPostArguments: Codable {
        let id: CustomUUID
        let title: String
        let tags: [Tag]
    }

    func editPost(request: Request, arguments: EditPostArguments) -> Post? {
        let postIndex = posts.firstIndex { $0.id == arguments.id }
        guard let index = postIndex?.indexValue else {
            return nil
        }

        posts[index].title = arguments.title
        posts[index].tags = arguments.tags
        return posts[index]
    }
}

The EditPostArguments structure has the three properties that we mentioned before. Then, the editPost function will accept those arguments as a parameter and will use the id to search for the Post . If we can’t find it, we will return nil to the client. If we manage to find it, we will update the properties of the Post with the values on the arguments and return the updated post to the client.

Last but not least, let’s see how we can add the functionality to create a new post entity!

Create mutation

To create a new Post we would need a title, a set of tags, and the id of the author.

It’s time to introduce a new GraphQL concept, the inputs. GraphQL distinguishes the types that can be used as input from those that can be used as outputs to queries. For example, ina previous post, I have used the type Author to return the author’s data. This is a typical example of an Output type. This kind of types, though, cannot be used when we want to pass arguments, be it in a query or a mutation. In order to define complex types for arguments, there is the concept of Input . Input is just like a type, with the only difference being the purpose of use and that it can only include scalar, enums, strings, int, float, bool, and other input types. We can not use an Output type as a field on an Input type.

So, let’s create a PostInput type and add the properties that we need to create a new Post .

struct PostInput: Codable {
    let title: String
    let tags: [Tag]
    let authorId: CustomUUID
}

Now, we can define the function to create a new Post on an extension of the PostController .

extension PostController {
    struct CreatePostArguments: Codable {
        let input: PostInput
    }
    func createPost(request: Request, arguments: CreatePostArguments) -> Post? {
        guard author.id == arguments.input.authorId else {
            return nil
        }
        let post = Post(
            id: CustomUUID(value: UUID()),
            title: arguments.input.title,
            publishedAt: Date(),
            tags: arguments.input.tags,
            author: author
        )
        posts.append(post)
        return post
    }
}

In the same way as for the other functions, we define a structure for the type of the arguments. This structure contains a sole field of the type PostInput that we defined earlier. Then the function createPost takes an instance of this CreatePostArguments structure and uses it to create a new Post entity, which we later append to the list of existing posts.

And that’s about it for the logic part of our mutations. Now, it’s time to integrate them into the GraphQL server.

GraphQL

To make those functions and arguments available on the GraphQL schema, we will have to update the FieldKeyProvider extension of the PostController and add the keys for them. We will use those keys to map the function and the arguments of PostController to the mutations and the arguments of the GraphQL schema.

enum FieldKeys: String {
+       case id
+       case title
+       case tags
+       case input
+
        case posts
+       case deletePost
+       case editPost
+       case createPost
    }

We will also provide conformance to the FieldKeyProvider protocol and define keys for the PostInput structure.

extension PostInput: FieldKeyProvider {
    typealias FieldKey = FieldKeys

    enum FieldKeys : String {
        case title
        case tags
        case authorId
    }
}

Lastly, we will update the GraphQL schema definition on Schema.swift by adding the definition for the PostInput input type and the definitions for the mutations using the keys we defined on the FieldKeyProvider extensions.

Query([
            Field(.posts, at: PostController.fetchPosts),
        ]),
+
+       Input(PostInput.self, [
+           InputField(.title, at: \.title),
+           InputField(.tags, at: \.tags),
+           InputField(.authorId, at: \.authorId)
+       ]),
+
+       Mutation([
+           Field(.deletePost, at: PostController.deletePost)
+               .argument(.id, at: \.id),
+
+           Field(.editPost, at: PostController.editPost)
+               .argument(.id, at: \.id)
+               .argument(.title, at: \.title)
+               .argument(.tags, at: \.tags),
+
+           Field(.createPost, at: PostController.createPost)
+               .argument(.input, at: \.input)
+       ])
+   ])

And that’s all folks! We can now build and run the vapor server!

How to test?

To verify what we have done so far, we are going to use a tool named GraphQL Playground .

The installation process is quite simple, you just have to run brew cask install graphql-playground . Then, you can use the Spotlight Search ( ⌘ + Space bar ) and open the application GraphQL Playground .

Once GraphQL Playground is running, we could run the following query to fetch the available posts:

query AllPosts {
  posts {
    id
    author {
        id
    }
  }
}

From the response, we are going to keep the id of the author and we will use it to create a new post with the following query (replace 00000000-0000-0000-0000-000000000000 with the UUID from the response):

mutation CreatePost {
  createPost(input: {
    authorId: {
      value: "00000000-0000-0000-0000-000000000000"
    },
    tags: [Swift, Vapor]
    title: "A new post"
  }) {
    id
    title
    publishedAt
    tags
    author {
      id
    }
  }
}

This time, keep the id of the post from the response, and use it on the next query to edit the title and the tags of the post:

mutation EditPost {
  editPost(
    id: {
      value: "00000000-0000-0000-0000-000000000000"
    },
    tags: [Swift, Vapor, GraphQL], 
    title: "A new post with an updated title") {
    id
    title
    publishedAt
    tags
  }
}

Finally, we can delete the post that we have created using the id of the post from the previous query on the following query:

mutation DeletePost {
  deletePost(id: {
    value: "00000000-0000-0000-0000-000000000000"
  })
}

Conclusion

And that’s about it! In this post, we have seen how to add mutations to create, edit and delete a Post on a GraphQL server built with Vapor. We have also seen how to take advantage of GraphQL’s Inputs for arguments with complex types and how to use GraphQL Playground to run our GraphQL queries.

In the next post, I am going to continue this GraphQL & Swift journey and I am going to investigate how to use the mutations that we defined in this post from an iOS app. So stay tuned and follow me on Twitter should you want to get notified once the next post is published or you have a question or comment about this post.

Thanks for reading this post, and see you next time!