Getting Started

Building GraphQL servers in golang
[edit]

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

You can find the finished code for this tutorial here

Note

This tutorial uses Go Modules and requires Go 1.11+. If you want to use this tutorial without Go Modules, take a look at our Getting Started Using dep guide instead.

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

Building the server

Define the schema

gqlgen is a schema-first library — before writing code, you describe your API using the GraphQL Schema Definition Language. This usually goes into a file called schema.graphql. A basic example as follows will be generated by the init command:

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!
}

Create the project skeleton

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

This has created an empty skeleton with all files you need:

Create the database models

The generated model for Todo isn’t right, it has a user embeded in it but we only want to fetch it if the user actually requested it. So instead lets make a new model in todo.go:

package gqlgen_todos

type Todo struct {
	ID     string
	Text   string
	Done   bool
	UserID string
}

Next tell gqlgen to use this new struct by adding it to gqlgen.yml:

models:
  Todo:
    model: github.com/[username]/gqlgen-todos.Todo

Regenerate by running:

$ go run github.com/99designs/gqlgen -v

Note

The verbose flag -v is here to show what gqlgen is doing. It has looked at all the fields on the model and found matching methods for all of them, except user. For user it has added a resolver to the interface you need to implement. This is the magic that makes gqlgen work so well!

Implement the resolvers

The generated runtime has defined an interface for all the missing resolvers that we need to provide. Lets take a look in generated.go:

func NewExecutableSchema(cfg Config) graphql.ExecutableSchema {}
	// ...
}

type Config struct {
	Resolvers  ResolverRoot
	// ...
}

type ResolverRoot interface {
	Mutation() MutationResolver
	Query() QueryResolver
	Todo() TodoResolver
}

type MutationResolver interface {
	CreateTodo(ctx context.Context, input NewTodo) (*Todo, error)
}
type QueryResolver interface {
	Todos(ctx context.Context) ([]Todo, error)
}
type TodoResolver interface {
	User(ctx context.Context, obj *Todo) (*User, error)
}

Notice the TodoResolver.User method? Thats gqlgen saying “I dont know how to get a User from a Todo, you tell me.”. Its worked out how to build everything else for us.

For any missing models (like NewTodo) gqlgen will generate a go struct. This is usually only used for input types and one-off return values. Most of the time your types will be coming from the database, or an API client so binding is better than generating.

Write the resolvers

This is a work in progress, we have a way to generate resolver stubs, but it cannot currently update existing code. We can force it to run again by deleting resolver.go and re-running gqlgen:

$ rm resolver.go
$ go run github.com/99designs/gqlgen

Now we just need to fill in the not implemented parts. Update resolver.go

package gqlgen_todos

import (
	context "context"
	"fmt"
	"math/rand"
)

type Resolver struct {
	todos []*Todo
}

func (r *Resolver) Mutation() MutationResolver {
	return &mutationResolver{r}
}
func (r *Resolver) Query() QueryResolver {
	return &queryResolver{r}
}
func (r *Resolver) Todo() TodoResolver {
	return &todoResolver{r}
}

type mutationResolver struct{ *Resolver }

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

type queryResolver struct{ *Resolver }

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

type todoResolver struct{ *Resolver }

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

We now have a working server, to start it:

go run server/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
      }
    }
}

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 ./...