SQLSTATE[23000]: Integrity constraint violation: in Laravel 5.2

I need pass Project Model id as project_id to My Task Model table. this is My TaskController

public function store(Request $request, Project $project)
{
    $task = new Task;
    $task->task_name   = $request->input('name');
    $task->body = $request->input('body');
    $task->assign = $request->input('status');
    $task->priority = $request->input('status');
     $task->duedate  = date("Y-m-d", strtotime($request->input("date")));


    // This will set the project_id on task and save it
    $project->tasks()->save($task);
}

this is My form action route regarding to store task data in projects folder blade file is show.blade.php

<form method="post" action="">

this is Task Mode

<?php

namespace App;

use App\User;
use App\Auth;
use App\Project;

use Illuminate\Database\Eloquent\Model;

class Task extends Model
{
     protected $fillable = ['task_name', 'body', 'assign','priority','duedate','project_id'];



      public function scopeProject($query, $id)
    {
        return $query->where('project_id', $id);
    }


     public function projects()
     {
         return $this->belongsTo('App\Project');
     }


    //
}

this is Project Model

 {
        protected $fillable = ['project_name','project_notes','project_status','color','group'];
        //

        public function tasks(){
             return $this->hasMany('App\Task');
    }


but I got this error massage here

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'project_id' cannot be null how can fix this problem?

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