can't using foreach for collection Laravel 5.3 - php

Question

I'm creating an edit page of a group-making of an organization. each group consists of a mentor and some mentees.
The problem is i can't iterate using the result because it is a collection of a collection and a user, instead of giving a (directly) collection of user.
the error is this
ErrorException in dd3280396f5fc9a234b8faac1b17b55bd5c30ae2.php line 77:
Undefined property: Illuminate\Database\Eloquent\Collection::$id (View: E:\Documents\Karya\simentor\resources\views\group\edit.blade.php)
Here's the result of dd($mentees)
Collection {#260 ▼
#items: array:2 [▼
0 => Collection {#286 ▼
#items: array:1 [▼
0 => User {#284 ▶}
]
}
1 => User {#261 ▶}
]
}
To get all mentors and mentees who don't have any group yet and to get the mentor and mentees of the to-be-edited group, i did this in GroupController.php
public function edit($id)
{
$group = Group::find($id);
//get mentor and mentee who don't have group yet
$unassignedMentors = User::whereHas(
'roles', function ($q) {
$q->where('name', 'mentor');
})
->doesntHave('groups')
->get();
$unassignedMentees = User::whereHas(
'roles', function ($q) {
$q->where('name', 'mentee');
})
->doesntHave('groups')
->get();
//get mentor and mentee of the to-be-edited group
$assignedMentor = $group->mentor();
$assignedMentee = $group->mentee();
//combine those mentors and mentees into a collection
$mentors = $unassignedMentors->prepend($assignedMentor);
$mentees = $unassignedMentees->prepend($assignedMentee);
return view('group.edit', compact('group', 'mentees', 'mentors', 'assignedMentor', 'assignedMentee'));
}
The mentor() and mentee() method defined in Group.php like this
public function mentor()
{
return $this->users()->whereHas(
'roles', function($q){
$q->where('name', 'mentor');
})
->first();
}
public function mentee()
{
return $this->users()->whereHas(
'roles', function($q){
$q->where('name', 'mentee');
})
->get();
}
Here's the code to display the checkboxes and names in edit.blade.php
#foreach ($mentees as $mentee)
<tr>
<td><input type="checkbox" name="mentee_id[]" value="{{$mentee->id}}" {{ array_has($assignedMentee, $mentee) ? 'checked' : '' }}></td> --> line 77
<td>{{ucwords($mentee->profile->name)}}</td>
</tr>
#endforeach