Add a relation manager widget to a form

Given are the following models (adapt the class names and namespaces to your usecase):

• Post: This is the base model, a simple blog post. We want to add the relation manager to this model's form.
• Comment: This is the related model. A blog post has many comments.

<?php
namespace Acme\Blog\Models;

class Post extends Model
{
    // The relation manager works for any relation type, use whatever you need.
    public $hasMany = ['comments' => \Acme\Blog\Models\Comment::class];
}

Implementing and configuring the relation controller

In your PostsController add the following lines:

<?php
namespace Acme\Blog\Controllers;

class Posts extends Controller
{
    public $implement = [
        'Backend.Behaviors.FormController',
        'Backend.Behaviors.RelationController',         // Add this!
    ];

    public $formConfig = 'config_form.yaml';
    public $relationConfig = 'config_relation.yaml';    // Add this!
}

Next, create the config_relation.yaml file in plugins/acme/blog/controllers/posts/config_relation.yaml.

Paste the contents below (in case of a hasMany relation) into the yaml file. You can find more examples for different relationship types (hasOne, belongsToMany, etc) in the official docs.

comments:
    label: Comment    
    deferredBinding: true
    view:
        list: $/acme/blog/models/comment/columns.yaml
        toolbarButtons: create|delete
    manage:
        form: $/acme/blog/models/comment/fields.yaml
        recordsPerPage: 10

Displaying the relation manager

Create a new file in plugins/acme/blog/controllers/posts/_comments.htm. Paste the following contents:

<?= $this->relationRender('comments'); ?>

The comments key corresponds to the base key in your config_relation.yaml.

Now you can include this partial in your model's form definition to display the relation manager.

# in plugins/acme/blog/models/post/fields.yaml
comments:
    type: partial
    path: comments # includes _comments.htm from the current controller's path.