error in symfony for manyToOne and form imbricated - symfony

Question

I encountered a problem under symfony3 which has blocked me for a while ... I do not understand this error:
Expected value of type "Doctrine\Common\Collections\Collection|array" for association field "AppBundle\Entity\Project#$participants", got "string" instead.
Here is my entity Project:
class Project
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #ORM\ManyToOne(targetEntity="AppBundle\Entity\User", inversedBy="projects")
* #ORM\JoinColumn(nullable=false)
*/
private $user;
/**
* #ORM\OneToMany(targetEntity="AppBundle\Entity\Participant", mappedBy="project", cascade={"persist", "remove"})
*/
private $participants;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255)
*/
private $title;
/**
* #var string
*
* #ORM\Column(name="content", type="text", nullable=true)
*/
private $content;
/**
* #var string
*
* #ORM\Column(name="place", type="string", length=255, nullable=true)
*/
private $place;
/**
* #var \DateTime
*
* #ORM\Column(name="dateEvent", type="datetime", nullable=true)
*/
private $dateEvent;
/**
* #var boolean
*
* #ORM\Column(name="status", type="boolean")
*/
private $status;
/**
* #var boolean
*
* #ORM\Column(name="in_progress", type="boolean")
*/
private $inProgress;
/**
* #var boolean
*
* #ORM\Column(name="accept_list", type="boolean")
*/
private $acceptList;
/**
* #var boolean
*
* #ORM\Column(name="visible_list", type="boolean")
*/
private $visibleList;
/**
* #var boolean
*
* #ORM\Column(name="many_loop", type="boolean")
*/
private $manyLoop;
/**
* #var string
*
* #ORM\Column(name="text_email", type="text", nullable=true)
*/
private $textEmail;
/**
* #var \DateTime
*
* #ORM\Column(name="created", type="datetime")
*/
private $created;
public function __construct()
{
$this->run = false;
$this->status = false;
$this->acceptList = true;
$this->visibleList = true;
$this->inProgress = false;
$this->manyLoop = true;
$this->created = new \Datetime('now');
$this->participants = new ArrayCollection();
}
/**
* Get id
*
* #return integer
*/
public function getId()
{
return $this->id;
}
/**
* Set title
*
* #param string $title
*
* #return Project
*/
public function setTitle($title)
{
$this->title = $title;
return $this;
}
/**
* Get title
*
* #return string
*/
public function getTitle()
{
return $this->title;
}
/**
* Set content
*
* #param string $content
*
* #return Project
*/
public function setContent($content)
{
$this->content = $content;
return $this;
}
/**
* Get content
*
* #return string
*/
public function getContent()
{
return $this->content;
}
/**
* Set place
*
* #param string $place
*
* #return Project
*/
public function setPlace($place)
{
$this->place = $place;
return $this;
}
/**
* Get place
*
* #return string
*/
public function getPlace()
{
return $this->place;
}
/**
* Set dateEvent
*
* #param \DateTime $dateEvent
*
* #return Project
*/
public function setDateEvent($dateEvent)
{
$this->dateEvent = $dateEvent;
return $this;
}
/**
* Get dateEvent
*
* #return \DateTime
*/
public function getDateEvent()
{
return $this->dateEvent;
}
/**
* Set status
*
* #param boolean $status
*
* #return Project
*/
public function setStatus($status)
{
$this->status = $status;
return $this;
}
/**
* Get status
*
* #return boolean
*/
public function getStatus()
{
return $this->status;
}
/**
* Set inProgress
*
* #param boolean $inProgress
*
* #return Project
*/
public function setInProgress($inProgress)
{
$this->inProgress = $inProgress;
return $this;
}
/**
* Get inProgress
*
* #return boolean
*/
public function getInProgress()
{
return $this->inProgress;
}
/**
* Set acceptList
*
* #param boolean $acceptList
*
* #return Project
*/
public function setAcceptList($acceptList)
{
$this->acceptList = $acceptList;
return $this;
}
/**
* Get acceptList
*
* #return boolean
*/
public function getAcceptList()
{
return $this->acceptList;
}
/**
* Set visibleList
*
* #param boolean $visibleList
*
* #return Project
*/
public function setVisibleList($visibleList)
{
$this->visibleList = $visibleList;
return $this;
}
/**
* Get visibleList
*
* #return boolean
*/
public function getVisibleList()
{
return $this->visibleList;
}
/**
* Set manyLoop
*
* #param boolean $manyLoop
*
* #return Project
*/
public function setManyLoop($manyLoop)
{
$this->manyLoop = $manyLoop;
return $this;
}
/**
* Get manyLoop
*
* #return boolean
*/
public function getManyLoop()
{
return $this->manyLoop;
}
/**
* Set textEmail
*
* #param string $textEmail
*
* #return Project
*/
public function setTextEmail($textEmail)
{
$this->textEmail = $textEmail;
return $this;
}
/**
* Get textEmail
*
* #return string
*/
public function getTextEmail()
{
return $this->textEmail;
}
/**
* Set created
*
* #param \DateTime $created
*
* #return Project
*/
public function setCreated($created)
{
$this->created = $created;
return $this;
}
/**
* Get created
*
* #return \DateTime
*/
public function getCreated()
{
return $this->created;
}
/**
* Set user
*
* #param \AppBundle\Entity\User $user
*
* #return Project
*/
public function setUser(\AppBundle\Entity\User $user)
{
$this->user = $user;
return $this;
}
/**
* Get user
*
* #return \AppBundle\Entity\User
*/
public function getUser()
{
return $this->user;
}
/**
* Add participant
*
* #param \AppBundle\Entity\Participant $participant
*
* #return Project
*/
public function addParticipant(\AppBundle\Entity\Participant $participant)
{
$this->participants[] = $participant;
// On lie l'annonce à la candidature
$participant->setProject($this);
return $this;
}
/**
* Remove participant
*
* #param \AppBundle\Entity\Participant $participant
*/
public function removeParticipant(\AppBundle\Entity\Participant $participant)
{
$this->participants->removeElement($participant);
}
/**
* Get participants
*
* #return \Doctrine\Common\Collections\Collection
*/
public function getParticipants()
{
return $this->participants;
}}
My Form:
$builder ->add('participants', ParticipantType::class);
/**
* {#inheritdoc}
*/
public function configureOptions (OptionsResolver $resolver)
{
$resolver->setDefaults(array(
'data_class' => 'AppBundle\Entity\Project',
'step' => 1,
));
}
Form imbricated :
public function buildForm(FormBuilderInterface $builder, array $options)
{
$builder
->add('lastName', TextType::class, array(
'required' => true,
'label' => 'Nom'
))
->add('firstName', TextType::class, array(
'required' => true,
'label' => 'Prénom'
))
->add('email', EmailType::class, array(
'required' => true,
'label' => 'Adresse mail'
));
}
/**
* {#inheritdoc}
*/
public function configureOptions(OptionsResolver $resolver)
{
$resolver->setDefaults(array(
'validation_groups' => array('registration')
));
}
So here is to summarize my problem, I do not understand the error that Symfony returns, yet from the examples on the internet I do not see why there is a problem here ...
If someone to a solution would be top, because I blocked on this problem for a long time ...
Thank's !

Answer 1

Make sure that you don't forget to add : use Doctrine\Common\Collections\ArrayCollection or use $this->participants = new \Doctrine\Common\Collections\ArrayCollection();

Answer 2

In your FormType you should configure the participants field as a collection of Participant. It should be done using CollectionType like this:
$builder->add('participants', CollectionType::class, array(
'entry_type' => ParticipantType::class,
'allow_add' => true,
...
));
EDIT
According to your data model, the participants Field is a collection of Participant object. As you can see in the error message you presented, an array is expected. The CollectionType will always return an array of the target type.
That is why I suggest you to consider using a CollectionType in your form type. You can learn more about CollectionType here and about Symfony forms in general here