Suppose we have a blog and we want to create post under a specific id to do that we need to validate our id using login and on the function we create a new object and under that object we put our data and to specify the specific id we put an user id

CreateController.php

    public function store(Request $request)
    {
        $this->validate($request, [
            'title' => 'required',
            'body' => 'required'
        ]);

        // Create Post
        $post = new Post;
        $post->title = $request->input('title');
        $post->body = $request->input('body');
        $post->user_id = auth()->user()->id;
        $post->save();

        return redirect('/posts')->with('success', 'Post Created');
    }

Post.php

class Post extends Model
{
    // Table Name 
    protected $table = 'posts';

    // primary key 
    public $primaryKey = 'id';
    // Timestamps
    public $timestamps = true;

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

<!--this is a model it shows that a post belongs to an user-->

User.php

class User extends Authenticatable
{
    use Notifiable;

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'name', 'email', 'password',
    ];

    /**
     * The attributes that should be hidden for arrays.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'remember_token',
    ];

    /**
     * The attributes that should be cast to native types.
     *
     * @var array
     */
    protected $casts = [
        'email_verified_at' => 'datetime',
    ];

    public function posts(){
        return $this->hasMany('App\Post');
    }
}
<!--this is also a model -->
<!--this shows us that an user can have many posts -->
<!--by the function posts-->