REST API: how to post a manytomany relationship with fosrestbundle - json

Question

In my Symfony 4 project, I have 2 entities: Question and PossibleAnswer. A question can have many possible answers and an answer can be used in many different questions.
I want to POST a question, I use FOSRestBundle. This is how I would format the body of my request:
{
"title": "my question",
"possibleAnswers": [
{
"id": 1,
"title": "my first answer"
}
]
}
At least this is how FOSRestBundle formats the answer when I create the data directly in the database and GET the question.
But I keep getting the same error:
Validation Failed, possibleAnswers This value is not valid
I use Symfony forms to validate my POST and the fieldType I use for the possibleAnswers field of Question is an EntityType with the option multiple set to true. Maybe that's where the error comes from.
So my question is: How do I format the body of my request to add a valid possibleAnswers field ? and if I am doing it right then, which form type should I use for it to work ?
Sorry for not adding any code, the context is actually quite more complex but I'm pretty sure either the JSON Format of my field or the form type is wrong.
[edit]
here are all the form types I tried for the possibleanswers field
//->add('possibleAnswers', EntityType::class, array('data' => [], 'required' => false, 'multiple' => true, 'class' => PossibleAnswer::class))
->add('possibleAnswers', CollectionType::class, array('required' => false, 'entry_type' => EntityType::class, 'entry_options' => array('class' => PossibleAnswer::class)))
//->add('possibleAnswers', CollectionType::class, array('required' => false, 'entry_type' => PossibleAnswerType::class))
//->add('possibleAnswers', CollectionType::class, array('required' => false, 'entry_type' => PossibleAnswerType::class, 'entry_options' => array('class' => PossibleAnswer::class)))
here are all the JSON formats I tried to select the possibleanswers:
{
"title": "a postman created question",
"answerConditions": [
{
"id": 1,
"title": "already existing question",
}
]
}
{
"title": "a postman created question",
"answerConditions": [
{
"possibleanswer_id": 1
}
]
}
{
"title": "a postman created question",
"answerConditions": [
{
"id": 1
}
]
}
{
"title": "a postman created question",
"answerConditions": [
1
]
}

Answer 1

If you want to create the PossibleAnswer when posting the Question, you need to replace in your form the EntityType by a CollectionType.
I often use it in my POST request when I want to post the sub-object (here the PossibleAnswer) when I post the parent (Question) :
Question form
$builder
->add('possibleAnswers', CollectionType::class, array(
'entry_type' => PossibleAnswerType::class,
'allow_add' => true,
))
;
PossibleAnswer form :
$builder
->add('title')
;
And the json looks like :
{
"title": "my question",
"possibleAnswers": [
{
"title": "my first answer"
}
]
}
Hope it can help.
Edit :
If you want to link a PossibleAnswer to a Question while creating you Question you can use this :
Question form :
$builder
->add('possibleAnswer', EntityType::class, array(
'class' => YOUR_ENTITY::class,
'multiple' => true
));
And you json should only contains the id of the entity you want to link to. It should be like this :
{
"title": "a postman created question",
"answerConditions": [
1
]
}