ent/doc/md/extension.md

---
id: extensions
title: Extensions
---

### Introduction

The Ent [Extension API](https://pkg.go.dev/entgo.io/ent/entc#Extension)
facilitates the creation of code-generation extensions that bundle together [codegen hooks](code-gen.md#code-generation-hooks),
[templates](templates.md) and [annotations](templates.md#annotations) to create reusable components
that add new rich functionality to Ent's core. For example, Ent's [entgql plugin](https://pkg.go.dev/entgo.io/contrib/entgql#Extension)
exposes an `Extension` that automatically generates GraphQL servers from an Ent schema.

### Defining a New Extension

All extension's must implement the [Extension](https://pkg.go.dev/entgo.io/ent/entc#Extension) interface:

```go
type Extension interface {
	// Hooks holds an optional list of Hooks to apply
	// on the graph before/after the code-generation.
	Hooks() []gen.Hook

	// Annotations injects global annotations to the gen.Config object that
	// can be accessed globally in all templates. Unlike schema annotations,
	// being serializable to JSON raw value is not mandatory.
	//
	//	{{- with $.Config.Annotations.GQL }}
	//		{{/* Annotation usage goes here. */}}
	//	{{- end }}
	//
	Annotations() []Annotation

	// Templates specifies a list of alternative templates
	// to execute or to override the default.
	Templates() []*gen.Template

	// Options specifies a list of entc.Options to evaluate on
	// the gen.Config before executing the code generation.
	Options() []Option
}
```
To simplify the development of new extensions, developers can embed [entc.DefaultExtension](https://pkg.go.dev/entgo.io/ent/entc#DefaultExtension)
to create extensions  without implementing all methods:

```go
package hello

// GreetExtension implements entc.Extension.
type GreetExtension struct {
	entc.DefaultExtension
}
```

### Adding Templates

Ent supports adding [external templates](templates.md) that will be rendered during
code generation. To bundle such external templates on an extension, implement the `Templates`
method:
```gotemplate title="templates/greet.tmpl"
{{/* Tell Intellij/GoLand to enable the autocompletion based on the *gen.Graph type. */}}
{{/* gotype: entgo.io/ent/entc/gen.Graph */}}

{{ define "greet" }}

{{/* Add the base header for the generated file */}}
{{ $pkg := base $.Config.Package }}
{{ template "header" $ }}

{{/* Loop over all nodes and add the Greet method */}}
{{ range $n := $.Nodes }}
    {{ $receiver := $n.Receiver }}
    func ({{ $receiver }} *{{ $n.Name }}) Greet() string {
		return "Hello, {{ $n.Name }}"
    }
{{ end }}

{{ end }}
```
```go
func (*GreetExtension) Templates() []*gen.Template {
	return []*gen.Template{
		gen.MustParse(gen.NewTemplate("greet").ParseFiles("templates/greet.tmpl")),
	}
}
```

### Adding Global Annotations

Annotations are a convenient way to supply users of our extension with an API 
to modify the behavior of code generation. To add annotations to our extension,
implement the `Annotations` method. Let's say in our `GreetExtension` we want
to provide users with the ability to configure the greeting word in the generated
code:

```go
// GreetingWord implements entc.Annotation.
type GreetingWord string

// Name of the annotation. Used by the codegen templates.
func (GreetingWord) Name() string {
	return "GreetingWord"
}
```
Then add it to the `GreetExtension` struct:
```go
type GreetExtension struct {
	entc.DefaultExtension
	word GreetingWord
}
```
Next, implement the `Annotations` method:
```go
func (s *GreetExtension) Annotations() []entc.Annotation {
	return []entc.Annotation{
		s.word,
	}
}
```
Now, from within your templates you can access the `GreetingWord` annotation:
```gotemplate
func ({{ $receiver }} *{{ $n.Name }}) Greet() string {
    return "{{ $.Annotations.GreetingWord }}, {{ $n.Name }}"
}
```

### Adding Hooks

The entc package provides an option to add a list of [hooks](code-gen.md#code-generation-hooks)
(middlewares) to the code-generation phase. This option is ideal for adding custom validators for the
schema, or for generating additional assets using the graph schema. To bundle
code generation hooks with your extension, implement the `Hooks` method:

```go
func (s *GreetExtension) Hooks() []gen.Hook {
    return []gen.Hook{
        DisallowTypeName("Shalom"),
    }
}

// DisallowTypeName ensures there is no ent.Schema with the given name in the graph.
func DisallowTypeName(name string) gen.Hook {
	return func(next gen.Generator) gen.Generator {
		return gen.GenerateFunc(func(g *gen.Graph) error {
			for _, node := range g.Nodes {
				if node.Name == name {
					return fmt.Errorf("entc: validation failed, type named %q not allowed", name)
				}
			}
			return next.Generate(g)
		})
	}
}
```

### Using an Extension in Code Generation

To use an extension in our code-generation configuration, use `entc.Extensions`, a helper
method that returns an `entc.Option` that applies our chosen extensions:

```go title="ent/entc.go"
//+build ignore

package main

import (
	"fmt"
	"log"

	"entgo.io/ent/entc"
	"entgo.io/ent/entc/gen"
)

func main() {
	err := entc.Generate("./schema",
		&gen.Config{},
		entc.Extensions(&GreetExtension{
			word: GreetingWord("Shalom"),
		}),
	)
	if err != nil {
		log.Fatal("running ent codegen:", err)
	}
}
```

### Popular Extensions

- **[elk (discontinued)](https://github.com/masseelch/elk)**  
  `elk` is an extension that generates RESTful API endpoints from Ent schemas. The extension generates HTTP CRUD
  handlers from the Ent schema, as well as an OpenAPI JSON file. By using it, you can easily build a RESTful HTTP server
  for your application. Please note, that `elk` has been discontinued in favor of `entoas`. An implementation generator
  is in the works.   
  Read [this blog post](https://entgo.io/blog/2021/07/29/generate-a-fully-working-go-crud-http-api-with-ent) on how to
  work with `elk`, and [this blog post](https://entgo.io/blog/2021/09/10/openapi-generator) on how to generate
  an [OpenAPI Specification](https://swagger.io/resources/open-api/).

- **[entoas](https://github.com/ent/contrib/tree/master/entoas)**
  `entoas` is an extension that originates from `elk` and was ported into its own extension and is now the official
  generator for and opinionated OpenAPI Specification document. You can use this to rapidly develop and document a
  RESTful HTTP server. There will be a new extension released soon providing a generated implementation integrating for
  the document provided by `entoas` using `ent`.
  
- **[entgql](https://github.com/ent/contrib/tree/master/entgql)**  
  This extension helps users build [GraphQL](https://graphql.org/) servers from Ent schemas. `entgql` integrates
  with [gqlgen](https://github.com/99designs/gqlgen), a popular, schema-first Go library for building GraphQL servers.
  The extension includes the generation of type-safe GraphQL filters, which enable users to effortlessly map GraphQL
  queries to Ent queries.   
  Follow [this tutorial](https://entgo.io/docs/tutorial-todo-gql) to get started.

- **[entproto](https://github.com/ent/contrib/tree/master/entproto)**  
  `entproto` generates Protobuf message definitions and gRPC service definitions from Ent schemas. The project also
  includes `protoc-gen-entgrpc`, a `protoc` (Protobuf compiler) plugin that is used to generate a working implementation
  of the gRPC service definition generated by Entproto. In this manner, we can easily create a gRPC server that can
  serve requests to our service without writing any code (aside from defining the Ent schema)!  
  To learn how to use and set up `entproto`, read [this tutorial](https://entgo.io/docs/grpc-intro). For more background
  you can read [this blog post](https://entgo.io/blog/2021/03/18/generating-a-grpc-server-with-ent),
  or [this blog post](https://entgo.io/blog/2021/06/28/gprc-ready-for-use/) discussing more `entproto` features.

- **[entviz](https://github.com/hedwigz/entviz)**  
  `entviz` is an extension that generates visual diagrams from Ent schemas. These diagrams visualize the schema in a web
  browser, and stay updated as we continue coding. `entviz` can be configured in such a way that every time we
  regenerate the schema, the diagram is automatically updated, making it easy to view the changes being made.  
  Learn how to integrate `entviz` in your project
  in [this blog post](https://entgo.io/blog/2021/08/26/visualizing-your-data-graph-using-entviz).