SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'post_id' cannot be null

我需要一些帮助...我正在根据教程制作一个标签函数，但是在我的数据库（post_tag）中，只保存了tag_id和id，没有保存post_id。在我的posts表中，创建帖子后我会收到帖子的id，但在post_tag中没有。你知道为什么吗...？

我的控制器

public function create(){
        $tags = Tag::all();
        return view('posts.create')->withTags($tags);
    }
public function store(Request $request )
    {
        $data = request()->validate([
            'caption' => 'required|max:255',
            'image' => 'required|image',
        ]);
        $post = new Post;
        $post->tags()->sync($request->tags, false);
        $imagePath = request('image')->store('uploads', 'public');
        $image = Image::make(public_path("storage/{$imagePath}"))->fit(1600, 1100);
        $image->save();
        auth()->user()->posts()->create([
            'caption' => $data['caption'],
            'image' => $imagePath,
        ]);
         return redirect('/profile/' . auth()->user()->id);
    }

我的create_post_tag_table

  public function up()
    {
        Schema::create('post_tag', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->integer('post_id')->unsigned();
            $table->foreign('post_id')->references('id')->on('posts');
            $table->integer('tag_id')->unsigned();
            $table->foreign('tag_id')->references('id')->on('tags');
        });
    }

我的create_posts_table

  public function up()
    {
        Schema::create('posts', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->unsignedBigInteger('user_id');
            $table->string('caption');
            $table->string('image');
            $table->timestamps();
            $table->index('user_id');
        });
    }