//controller
<?php
public function add()
    {
        $this->loadModel('DogsCats');

        $dogsUnmated = $this->DogsCats->Dogs->find('list')->notMatching('DogsCats');
        $this->set('dogsUnmated', $dogsUnmated);

        $catsUnmated = $this->DogsCats->Cats->find('list')->notMatching('DogsCats');
        $this->set('catsUnmated', $catsUnmated);

        //real addition

        $farms = $this->DogsCats->newEntity();
        if ($this->request->is('post')) {
            $farms = $this->DogsCats->patchEntity($farms, $this->request->getData());

            if ($this->DogsCats->save($farms)) {
                $this->Flash->success(__('Hai aggiunto il tuo cane con il gatto.',['key'=>'message']));
                return $this->redirect(['action' => 'index']);
            }
            $this->Flash->error(__('Unable to add your allenatore.'));
        }
        $this->set('farms', $farms);

    }

//add.ctp
<?php echo $this->Html->script(['single']); ?>
<?php echo $this->Form->create($farms); ?>
<fieldset>
     <h1>Aggiungi cani e gatti</h1>

    <div class="form-group">
        <div class="col-lg-10 col-lg-offset-2">
        <?php
        //how can I make the value field dynamic? avoiding that I always overwrite the data with id 1
        echo $this->Form->control('id', ['type' => 'hidden', 'value' => 1]);  ?>
        </div>
    </div>
    <div class="form-group">
        <div class="col-lg-10 col-lg-offset-2">
         <label for="info">Dog</label>
         <?php echo $this->Form->select('dog_id', $dogsUnmated,['class'=>'form-control','required' => true]);?>
        </div>
    </div>
    <div class="form-group">
        <div class="col-lg-10 col-lg-offset-2">
        <label for="info">Cat</label>
        <?php echo $this->Form->select('dog_id', $catsUnmated,['class'=>'form-control','required' => true]);?>
        </div>
    </div>
    <div class="form-group">
        <div class="col-lg-10 col-lg-offset-2">
         <?php echo $this->Form->control('info',['class'=>'form-control','required' => true]);?>
        </div>
    </div>
    <div class="form-group">
        <div class="col-lg-10 col-lg-offset-2">
         <?php echo $this->Form->button(__('Save'),['class'=>'btn btn-primary']);?>
        </div>
    </div>
</fieldset>
<?php echo $this->Form->end();?>
<?php echo $this->Html->link('back', ['action'=>'index'], ['class'=>'btn btn-primary']) ?>