linkTitle	title	description	menu	weight
Getting Started	Building GraphQL servers in golang	Get started building type-safe GraphQL servers in Golang using gqlgen	main	-7

This tutorial will take you through the process of building a GraphQL server with gqlgen that can:

• Return a list of todos
• Create new todos
• Mark off todos as they are completed

You can find the finished code for this tutorial here

Setup Project

Create a directory for your project, and initialise it as a Go Module:

$ mkdir gqlgen-todos
$ cd gqlgen-todos
$ go mod init github.com/[username]/gqlgen-todos
$ go get github.com/99designs/gqlgen

Building the server

Create the project skeleton

$ go run github.com/99designs/gqlgen init

This will create our suggested package layout. You can modify these paths in gqlgen.yml if you need to.

├── go.mod
├── go.sum
├── gqlgen.yml               - コード生成を制御するためのコンフィグファイル
├── graph
│   ├── generated            - A package that only contains the generated runtime
│   │   └── generated.go
│   ├── model                - A package for all your graph models, generated or otherwise
│   │   └── models_gen.go
│   ├── resolver.go          - The root graph resolver type. This file wont get regenerated
│   ├── schema.graphqls      - Some schema. You can split the schema into as many graphql files as you like
│   └── schema.resolvers.go  - the resolver implementation for schema.graphql
└── server.go                - The entry point to your app. Customize it however you see fit

Define your schema

gqlgen is a schema-first library — before writing code, you describe your API using the GraphQL Schema Definition Language. By default this goes into a file called schema.graphql but you can break it up into as many different files as you want.

The schema that was generated for us was:

type Todo {
  id: ID!
  text: String!
  done: Boolean!
  user: User!
}

type User {
  id: ID!
  name: String!
}

type Query {
  todos: [Todo!]!
}

input NewTodo {
  text: String!
  userId: String!
}

type Mutation {
  createTodo(input: NewTodo!): Todo!
}

Implement the resolvers

gqlgen generate compares the schema file (graph/schema.graphqls) with the models graph/model/* and wherever it can it will bind directly to the model.

If we take a look in graph/schema.resolvers.go we will see all the times that gqlgen couldn't match them up. For us it was twice:

func (r *mutationResolver) CreateTodo(ctx context.Context, input model.NewTodo) (*model.Todo, error) {
	panic(fmt.Errorf("not implemented"))
}

func (r *queryResolver) Todos(ctx context.Context) ([]*model.Todo, error) {
	panic(fmt.Errorf("not implemented"))
}

We just need to implement these two methods to get our server working:

First we need somewhere to track our state, lets put it in graph/resolver.go:

type Resolver struct{
	todos []*model.Todo
}

This is where we declare any dependencies for our app like our database, it gets initialized once in server.go when we create the graph.

func (r *mutationResolver) CreateTodo(ctx context.Context, input model.NewTodo) (*model.Todo, error) {
	todo := &model.Todo{
		Text:   input.Text,
		ID:     fmt.Sprintf("T%d", rand.Int()),
		User: &model.User{ID: input.UserID, Name: "user " + input.UserID},
	}
	r.todos = append(r.todos, todo)
	return todo, nil
}

func (r *queryResolver) Todos(ctx context.Context) ([]*model.Todo, error) {
	return r.todos, nil
}

We now have a working server, to start it:

go run server.go

then open http://localhost:8080 in a browser. here are some queries to try:

mutation createTodo {
  createTodo(input: { text: "todo", userId: "1" }) {
    user {
      id
    }
    text
    done
  }
}

query findTodos {
  todos {
    text
    done
    user {
      name
    }
  }
}

Don't eagerly fetch the user

This example is great, but in the real world fetching most objects is expensive. We dont want to load the User on the todo unless the user actually asked for it. So lets replace the generated Todo model with something slightly more realistic.

Create a new file called graph/model/todo.go

package model

type Todo struct {
	ID     string `json:"id"`
	Text   string `json:"text"`
	Done   bool   `json:"done"`
	UserID string `json:"user"`
}

Note

By default gqlgen will use any models in the model directory that match on name, this can be configured in gqlgen.yml.

And run go run github.com/99designs/gqlgen generate.

Now if we look in graph/schema.resolvers.go we can see a new resolver, lets implement it and fix CreateTodo.

func (r *mutationResolver) CreateTodo(ctx context.Context, input model.NewTodo) (*model.Todo, error) {
	todo := &model.Todo{
		Text:   input.Text,
		ID:     fmt.Sprintf("T%d", rand.Int()),
		UserID: input.UserID, // fix this line
	}
	r.todos = append(r.todos, todo)
	return todo, nil
}

func (r *todoResolver) User(ctx context.Context, obj *model.Todo) (*model.User, error) {
	return &model.User{ID: obj.UserID, Name: "user " + obj.UserID}, nil
}

Finishing touches

At the top of our resolver.go, between package and import, add the following line:

//go:generate go run github.com/99designs/gqlgen

This magic comment tells go generate what command to run when we want to regenerate our code. To run go generate recursively over your entire project, use this command:

go generate ./...