Car.php

<?php

[...]

class Car
{
    /**
     * @ORM\OneToMany(targetEntity="Condition", mappedBy="Car", cascade={"all"}, orphanRemoval=true)
     */
    private $conditions;

    public function setConditions($conditions)
    {
        $this->conditions = $conditions;
    }

    public function getConditions()
    {
        return $this->conditions;
    }

    public function addCondition($condition)
    {
        $this->conditions->add($condition);
        $condition->setCar($this);
    }

    public function removeCondition($condition)
    {
        $this->conditions->remove($condition);
    }

CarFormType.php

<?php 
[...]
                $builder->add('conditions', 'collection', array(
                        'type' => new ConditionType(),
                        'allow_add' => true,
                        'single_control' => false,
                        'allow_delete' => true,
                        'show_legend' => false,
                        'prototype' => true,
                        'options' => array(
                            'data_class' => 'Condition',
                            'label_render' => false,
                            'required' => false,
                            'widget_control_group' => false
                        )
                ));

Condition.php

<?php

[...]

class Condition
{
    /**
     * @var integer $id
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string $name
     *
     * @ORM\Column(name="name", type="string", length=255)
     */
    private $name;

    /**
     * @var integer $car
     *
     * @ORM\ManyToOne(targetEntity="Car", inversedBy="conditions")
     */
    private $car;


    [getters and setters]
}

ConditionType.php

<?php 

class ConditionType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder->add('name');
    }

    public function getName()
    {
       return 'condition';
    }

    public function getDefaultOptions()
    {
    	return array('data_class' => '[...]\Condition');
    }
}