set a field in pivot table whenever a record insert or update happens in laravel - eloquent

Question

OK, recently I am implementing an RBAC based system in laravel.
I have these two classes as my models:
class User extends Authenticatable
{
public function roles(){
return $this->belongsToMany(\App\Role::class, 'user_role', 'user_id', 'role_id')->withPivot('is_expired', 'assigned_at', 'expire_at');
}
}
class Role extends Model
{
public function users(){
return $this->belongsToMany(\App\User::class, 'user_role', 'role_id', 'user_id')->withPivot('is_expired', 'assigned_at', 'expire_at');
}
}
it works fine BUT, I want to set a default value for the expire_at attribute of the pivot table based on an attribute of the Role model. for example I have a period attribute on Roles table, and its a number representing number of months.
So i want when a role assigned to a user (inserted in pivot table) the value of expire_at set to currentDatetime + thePeriodNum months and save in the pivot table.
how can I achieve this?
I have tried laravel custom pivot class and mutators but it seems not working or I did something wrong.
somebody mentioned that the mutators dont get triggered when using attach()/detach() methods so I think even if it was working i could not see the difference.
Someone mentioned its possible with observers but I have no Idea what is an observer im noob.
So thats all, it would be really good for me if anybody could help me through this mess I'm in right now.
Thanks in advance.

Answer 1

It is possible to attach the new role and set the expires_at column at the same time. This would avoid needing to use observers (listeners of model events) within your code.
The code would look like the following:
$role = Role::find($roleId);
$expireAt = now()->addMonths($role->period)->toDateTimeString();
// or Carbon::now()->addMonths($role->period)->toDateTimeString();
User::find($userId)->roles()->attach($role->id, ['expire_at' => $expireAt]);
Here, the role is found. A timestamp is created by taking the current time, adding the addition months based on the role's period (this should be an integer value).
Finally, this is added to the attachment of the role to the user.
Add as a model method
This can all be added as a function/method on the User model which would clean up the code into one action, ->addRole($roleId):
// Within App\User.php
public function addRole($roleId)
{
$role = \App\Role::find($roleId);
$expiresAt = now()->addMonths($role->period)->toDateTimeString();
$this->roles()->attach($role->id, [ 'expire_at' => $expiresAt ]);
}
This can then be called with the following:
$user = User::find($id);
$user->addRole($roleId);
I hope this help.