ent

Setup A Go Environment

❯ go version
go version go1.16.6 darwin/amd64

❯ go mod init github.com/k3forx/ent

Installation

❯ go get entgo.io/ent/cmd/ent

Create Your First Schema

❯ go run entgo.io/ent/cmd/ent init User

The above command creates the following file.

package schema

import "entgo.io/ent"

// User holds the schema definition for the User entity.
type User struct {
        ent.Schema
}

// Fields of the User.
func (User) Fields() []ent.Field {
        return nil
}

// Edges of the User.
func (User) Edges() []ent.Edge {
        return nil
}

Add 2 fields to the User schema:

package schema

// Fields of the User.
func (User) Fields() []ent.Field {
    return []ent.Field{
        field.Int("age").
            Positive(),
        field.String("name").
            Default("unknown"),
    }
}

Run go generate from the root directory of the project as follows:

❯ go generate ./ent

This produces the following files:

❯ tree ./ent
./ent
├── client.go
├── config.go
├── context.go
│   └── user.go
... truncated
├── tx.go
├── user
│   ├── user.go
│   └── where.go
├── user.go
├── user_create.go
├── user_delete.go
├── user_query.go
└── user_update.go

7 directories, 22 files

Create Your First Entity

To get started, create a new ent.Client. For this example, we will use MySQL.

main.go

package main

import (
        "context"
        "fmt"
        "log"

        "github.com/k3forx/ent/ent"

        _ "github.com/go-sql-driver/mysql"
)

func main() {
        client, err := ent.Open("mysql", "mysql:mysql@tcp(localhost:3306)/test?parseTime=True")
        if err != nil {
                log.Fatalf("failed opening connection to mysql: %v", err)
        }
        defer client.Close()

        // Run the auto migration tool
        if err := client.Schema.Create(context.Background()); err != nil {
                log.Fatalf("failed creating schema resources: %v", err)
        }
}

Now, we're ready to create our user. Let's call this function CreateUser for the sake of example:

func CreateUser(ctx context.Context, client *ent.Client) (*ent.User, error) {
        u, err := client.User.Create().SetAge(30).SetName("a8m").Save(ctx)
        if err != nil {
                return nil, fmt.Errorf("failed creating user: %w", err)
        }
        log.Println("user was created: ", u)
        return u, nil
}

Run the script

❯ go run ./main.go
2021/08/11 23:36:19 user was created:  User(id=1, age=30, name=a8m)
user: User(id=1, age=30, name=a8m)%

Add Your First Edge (Relation)

We want to declare an edge (relation) to another entity in the schema. Let's create two additional entities named Car and Group with a few fields. We use ent CLI to generate this initial schemas:

❯ go run entgo.io/ent/cmd/ent init Car Group

And then we add the rest of the fields manually:

./ent/schema/car.go

// Fields of the Car.
func (Car) Fields() []ent.Field {
	return []ent.Field{
		field.String("model"),
		field.Time("registered_at"),
	}
}

ent/schema/group.go

// Fields of the Group.
func (Group) Fields() []ent.Field {
    return []ent.Field{
        field.String("name").
            // Regexp validation for group name.
            Match(regexp.MustCompile("[a-zA-Z_]+$")),
    }
}

Let's define our first relation. An edge from User to Car defining that a user can have 1 or more cars, but a car has only one owner (one-to-many relation).

Let's add the "cars" edge to the User schema, and run go generation ./ent:

ent/schema/user.go

// Edges of the User.
func (User) Edges() []ent.Edge {
    return []ent.Edge{
        edge.To("cars", Car.Type),
    }
}

We continue our example by creating 2 cars and adding them to a user.

main.go

func CreateCars(ctx context.Context, client *ent.Client) (*ent.User, error) {
    // Create a new car with model "Tesla".
    tesla, err := client.Car.
        Create().
        SetModel("Tesla").
        SetRegisteredAt(time.Now()).
        Save(ctx)
    if err != nil {
        return nil, fmt.Errorf("failed creating car: %w", err)
    }
    log.Println("car was created: ", tesla)

    // Create a new car with model "Ford".
    ford, err := client.Car.
        Create().
        SetModel("Ford").
        SetRegisteredAt(time.Now()).
        Save(ctx)
    if err != nil {
        return nil, fmt.Errorf("failed creating car: %w", err)
    }
    log.Println("car was created: ", ford)

    // Create a new user, and add it the 2 cars.
    a8m, err := client.User.
        Create().
        SetAge(30).
        SetName("a8m").
        AddCars(tesla, ford).
        Save(ctx)
    if err != nil {
        return nil, fmt.Errorf("failed creating user: %w", err)
    }
    log.Println("user was created: ", a8m)
    return a8m, nil
}

But what about querying the cars edge (relation)? Here's how we do it:

main.go

func QueryCars(ctx context.Context, a8m *ent.User) error {
    cars, err := a8m.QueryCars().All(ctx)
    if err != nil {
        return fmt.Errorf("failed querying user cars: %w", err)
    }
    log.Println("returned cars:", cars)

    // What about filtering specific cars.
    ford, err := a8m.QueryCars().
        Where(car.Model("Ford")).
        Only(ctx)
    if err != nil {
        return fmt.Errorf("failed querying user cars: %w", err)
    }
    log.Println(ford)
    return nil
}

Add Your First Inverse Edge (BackRef)

Assume we have a Car object and we want to get its owner; the user that this car belongs to. For this, we have another type of edge called "inverse edge" that is defined using the edge.Form function.

The new edge created in the diagram above is translucent, to emphasize that we don't create another edge in the database. It's just a back-reference to the real edge (relation).

Let's add an inverse edge named owner to the Car schema, reference it to the cars edge in the User schema, and run go generate ./ent.

ent/schema/car.go

// Edges of the Car.
func (Car) Edges() []ent.Edge {
    return []ent.Edge{
        // Create an inverse-edge called "owner" of type `User`
        // and reference it to the "cars" edge (in User schema)
        // explicitly using the `Ref` method.
        edge.From("owner", User.Type).
            Ref("cars").
            // setting the edge to unique, ensure
            // that a car can have only one owner.
            Unique(),
    }
}

We'll continue the user/cars example above by querying the inverse edge.

main.go

func QueryCarUsers(ctx context.Context, a8m *ent.User) error {
    cars, err := a8m.QueryCars().All(ctx)
    if err != nil {
        return fmt.Errorf("failed querying user cars: %w", err)
    }
    // Query the inverse edge.
    for _, ca := range cars {
        owner, err := ca.QueryOwner().Only(ctx)
        if err != nil {
            return fmt.Errorf("failed querying car %q owner: %w", ca.Model, err)
        }
        log.Printf("car %q owner: %q\n", ca.Model, owner.Name)
    }
    return nil
}

Create Your Seconde Edge

We'll continue our example by creating a M2M (many-to-many) relationship between users and groups.

As you can see, each group entity can have many users, and a user can be connected to many groups; a simple "many-to-many" relationship. In the above illustration, the Group schema is the owner of the users edge (relation), and the User entity has a back-reference/inverse edge to this relationship named groups. Let's define this relationship in our schemas:

ent/schema/group.go

// Edges of the Group.
func (Group) Edges() []ent.Edge {
   return []ent.Edge{
       edge.To("users", User.Type),
   }
}

ent/schema/user.go

// Edges of the User.
func (User) Edges() []ent.Edge {
   return []ent.Edge{
       edge.To("cars", Car.Type),
       // Create an inverse-edge called "groups" of type `Group`
       // and reference it to the "users" edge (in Group schema)
       // explicitly using the `Ref` method.
       edge.From("groups", Group.Type).
           Ref("users"),
   }
}

We run ent on the schema directory to re-generate the assets.

❯ go generate ./ent

k3forx / ent

ent