Can not retrieve the id of a foreign key [Laravel] - php

Question

I have 2 tables: User and demands. A User can have as many demands as he wants. Inside the demands table, there is the User foreign key.
When the User completes the form, I put the User & demands information inside those 2 tables.
But I do not know how to have the User id to put it inside the demands table.
Here is the function inside my Controller:
public function store(Request $request){
$demand = new Demand;
$user = new user ;
$user ->numStudent= $request->NumStudent;
$user ->role_id = "3";
$user ->Lastname = $request->LastName;
$user ->FirstName= $request->FirstName;
$user ->numero_etudiant = "12345678";
$user ->email = $request->Email;
$user ->password = bcrypt('idk');
$user ->adress= $request->Adress;
$user ->phone = $request->Phone;
$user ->parcours = $request->Parcours;
$demand->status_id= "3";
//problem below
$demand->User_id= $User;
//
$demand->time_id = $request->LoanTime;
$demand->creation_date = $request->CreationDate;
$demand->comments = "";
$user ->save();
$demand->save();
return redirect('/demands_list');
}
But it says "Can't use method return value in write context".
Cordially

Answer 1

You have to save first the instance of your user before using its properties. Your code should look like this.
public function store(Request $request){
$demand = new Demand;
$user = new user ;
$user->numStudent= $request->NumStudent;
$user->role_id = "3";
$user->Lastname = $request->LastName;
$user->FirstName= $request->FirstName;
$user->numero_etudiant = "12345678";
$user->email = $request->Email;
$user->password = bcrypt('idk');
$user->adress= $request->Adress;
$user->phone = $request->Phone;
$user->parcours = $request->Parcours;
$user->save();
$demand->status_id= "3";
//problem below
$demand->User_id= $user->id;
//
$demand->time_id = $request->LoanTime;
$demand->creation_date = $request->CreationDate;
$demand->comments = "";
$demand->save();
return redirect('/demands_list');
}

Answer 2

Assuming by your tags that you are looking for an Eloquent (i.e. Laravel) way of doing this.
The short answer: use route model binding and keep your controller methods organized and single-focused.
As already answered, your User needs to be saved first. I would recommend you do this step in its own controller:
// UsersController.php
public function store(Request $request) {
$attributes = $request->validate([
// validation rules
]);
$user = User::create($attributes);
return redirect('/users/' . $user->id);
}
Now that you have a User created, you can then attach related models. Create a separate set of routes pointing to a separate controller for handling the relationship:
// routes/web.php
Route::get('/user/{user}/demands', 'UserDemandsController#index');
Route::post('/user/{user}/demands', 'UserDemandsController#store');
// ^^^^^^
// used with route model binding
Here's where route model binding comes into play, loading the model straight from the route parameters ({user}):
// UserDemandsController.php
public function store(Request $request, User $user) { // <-- ROUTE MODEL BINDING
$attributes = $request->validate([
// validation rules
]);
$user->demands()->create($attributes);
}
The result is very clean, simple code.