post.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Post extends Model
{
    public function tags()
    {
      return $this->belongsToMany('App\Tag');
    }

}

PostController.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;

use App\Http\Requests;

use Session;

use App\Post;
use App\Tag;
use App\Category;

class PostController extends Controller
{
  /**
  * Store a newly created resource in storage.
  *
  * @param  \Illuminate\Http\Request  $request
  * @return \Illuminate\Http\Response
  */
  public function store(Request $request)
  {
    $this->validate($request, array(
      'title'       => 'required|max:255',
      'slug'        => 'required|alpha_dash|min:5|max:255|unique:posts,slug',
      'category_id' => 'required|integer',
      'body'        => 'required'
    ));

    $post = new Post;
    $post->title = $request->title;
    $post->slug = $request->slug;
    $post->category_id = $request->category_id;
    $post->body = $request->body;

    $post->save();

    $post->tags()->sync($request->tags_id, false);

    Session::flash('success','Post gravado com sucesso!');

    return redirect()->route('posts.show', $post->id);
  }

}

tag.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Tag extends Model
{
    public function Posts()
    {
      return $this->belongsToMany('App\Post');
    }
}