Getting "Integrity constraint violation: 1452" in Laravel

Question

I am using two pivot tables to store two different relations of a table. One relation works fine on insert. but the next one throws the following error. I don't know what's causing this.

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (sdesk.approvers, CONSTRAINT approvers_user_id_foreign FOREIGN KEY (user_id) REFERENCES users (id)) (SQL: insert into approvers (user_id, request_id, updated_at, created_at) values (2, 6, 2014-04-29 10:54:37, 2014-04-29 10:54:37))

Here's my code:

UrequestsController.php

 public function postCreate() { 
    $request = new Urequest;
    $request->vms = Input::get('vms');
    $request->location = Input::get('location');
    $request->descr = Input::get('descr');
    $request->status = Input::get('status');
    //$request->role_group = Input::get('team');
    $request->save();

    $ruser = new Ruser;
    $ruser->user_id = Input::get('userid');
    $ruser->urequest()->associate($request);
    $ruser->save();

    $approver = new Approver;
    $approver->user_id = Input::get('aid');
    $approver->urequest()->associate($request);
    $approver->save();


    return Redirect::to('users/dashboard')->with('message', 'Saved!');
}

Approver.php

 class Approver extends Eloquent {


protected $fillable = array('request_id', 'user_id');


public function urequest() {
    return $this->belongsTo('Urequest','request_id'); 
}

 }

Ruser.php

 class Ruser extends Eloquent {


protected $fillable = array('request_id', 'user_id');


public function urequest() {
    return $this->belongsTo('Urequest','request_id');
}

 }

urequest.php

 class Urequest extends Eloquent {

protected $table = 'requests';
protected $fillable = array('vms', 'location', 'descr', 'status');

public function ruser() {
    return $this->hasOne('Ruser'); 
}

public function approver() {
    return $this->hasOne('Approver'); 
}

 } 

Schema

 Schema::create('requests', function($table)
{
            $table->increments('id');
            $table->string('vms', 20);
            $table->string('location', 20);
                    $table->string('descr', 255);
            $table->string('status', 20);
            $table->timestamps();
        });

 Schema::create('rusers', function($table)
    {
        $table->increments('id');
        $table->integer('request_id')->unsigned();
        $table->foreign('request_id')->references('id')->on('requests');
        $table->integer('user_id')->unsigned();
        $table->foreign('user_id')->references('id')->on('users');
        $table->timestamps();
    });

  Schema::create('approvers', function($table)
    {
        $table->increments('id');
        $table->integer('request_id')->unsigned();
        $table->foreign('request_id')->references('id')->on('requests');
        $table->integer('user_id')->unsigned();
        $table->foreign('user_id')->references('id')->on('users');
        $table->timestamps();
    });

Answer 1

"A foreign key constraint" usually fails when you don't have a row for the value(s) you are inserting/updating in the foreign table.

In this case you probably doesn't have a user_id of 2 in the users table or a request_id of 6 in the requests table.

EDIT just saw morawcik answered it in comments.