laravelで中間テーブルにデータが保存されない

記事投稿の際にあらかじめ用意されているタグ一覧からcheckboxで選択し記事を投稿できるように実装したつもりですが、中間テーブルに保存されません。

php
1    posts_table    
2
3    public function up()
4    {
5        Schema::create('posts', function (Blueprint $table) {
6            $table->bigIncrements('id');
7            $table->string('body');
8            $table->timestamps();
9        });
10    }

php
1    tags_table    
2
3    public function up()
4    {
5        Schema::create('tags', function (Blueprint $table) {
6            $table->bigIncrements('id');
7            $table->string('name');
8            $table->timestamps();
9        });
10    }

php
1    post_tag_table
2
3    public function up()
4    {
5        Schema::create('post_tag', function (Blueprint $table) {
6            $table->bigIncrements('id');
7            $table->unsignedBigInteger('post_id');
8            $table->foreign('post_id')->references('id')->on('posts');
9            $table->unsignedBigInteger('tag_id');
10            $table->foreign('tag_id')->references('id')->on('tags');
11            $table->timestamps();
12        });
13    }

php
1    controller
2
3    public function create(Request $request)
4    {
5        $validatedData = $request->validate([
6            'body' => 'required'
7        ]);
8        
9        $post = new Post();
10        $post->body = $request->body;
11        $post->save();
12        $post->tags()->attach($request->tags);
13        
14        return redirect('admin/post/create');
15    }

php
1     blade
2
3      <div class="form-group">
4      <label for="body">投稿</label>
5           <textarea class="form-control" name="body" rows="20">{{ old('body') }}</textarea>
6      </div>
7      {{ csrf_field() }}
8      @foreach ($tags as $tag)
9          <input type="checkbox" name="tag[]" value="{{ $tag->id }}">{{ $tag->name }}
10      @endforeach
11      <input type="submit" class="btn btn-primary" value="作成">