Branch.php

<?php

namespace forest\Model;


class Branch extends \forest\Model
{
    public $table = 'branch';
    public $title_field = 'branch_name';
    public $caption = 'Branch';
    function init() {
        parent::init();
        $this->addField('name');
        $this->hasMany('TreeBranch', new TreeBranch());
    }
}

Tree.php

<?php
namespace forest\Model;

class Tree extends \forest\Model
{
    public $table = 'tree';
    public $title_field = 'tree_name';
    public $caption = 'Tree';

    function init() {
        parent::init();
        $this->addField('name');
        $this->addField('branches_ref');
        $this->hasMany('TreeBranch', new TreeBranch());
        $this->addHook(
            'afterSave',
            function ($new_tree)
            {
              $this->setRef('branches',$new_tree);
            }
        );
    }

    public function setRef($key,$new_tree){
        if($key === 'branches'){
          // new branch ids
          $last = json_decode($new_tree->get('branches_ref'));
          $saved = $new_tree->ref('TreeBranch')->export();
          $add = $delete = $same = [];
          foreach($saved as $key => $set) {
            if(in_array($set['branch_id'],$last)) {
              $same[] = $set['branch_id'];
            }else{
              $delete[] = $set['branch_id'];
            }
          }
          $add = array_diff($last,$same);
          // update with add and delete
          if(count($delete)){
            $tree->ref('TreeBranch')->addCondition('branch_id','in',$delete)->action('delete')->execute();
          }
          if(count($add)){
            $add_ref = [];
            foreach($add as $branch_id){
              $add_ref[] = ['tree_id'=>$tree->get('id'),'branch_id'=>$branch_id];
            }
            $tree->refModel('TreeBranch')->import($add_ref);
          }
        }
    }
}

tree_add.php

<?php

$app = new forest\App();
$form = $app->add(
    [
        '\forest\Form',
        'onCancelButtonClick' => $app->jsRedirect(['/trees']),
        'onSuccess' => function ($f) {
            return [
                $f->success('Tree created successfully'),
                $f->app->jsRedirect(['/trees'])
            ];
        }
    ]
);
$has_branches = isset($_REQUEST['branches']) && is_array($_REQUEST['branches']);
$form->setModel($tree_model, []);
if ($has_branches) {
  $branch_model = new \forest\Model\Branch($app->db);
  $branch_model->set('branches_ref',json_encode($_REQUEST['branches']));
  $form->addField('branches_ref',['Hidden']);
}
$form->addField('name')

TreeBranch.php

<?php

namespace forest\Model;


class TreeBranch extends \forest\Model
{
    public $table = 'tree_branch';
    public $caption = 'Tree Branch Relation';

    function init()
    {
        parent::init();


        $this->hasOne('tree_id', [new Tree(), 'ui'=>['visible'=>false]])->addTitle();
        $this->hasOne('branch_id', [new Branch(), 'ui'=>['visible'=>false]])->addTitle();
    }
}