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

Question

I need some help with this...I'm making a function for tags with a tutorial, but in my database ( post_tag ), my post_id isn't saved, only tag_id and id.In my posts table, after create a post I receive the id of post, but here, in my post_tag isn't.Do you know why guys...?
My controller
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);
}
My 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');
});
}
My 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');
});
}

Answer 1

You have created a new Post object, but you have not saved it yet in this line:
$post = new Post;
Thus, in this line, immediately following:
$post->tags()->sync($request->tags, false);
There is no id in the database for this post as yet (nor will there be an id on the Post model), and thus sync will fail every time because it can't find the id.
After you new up the Post object, save() it and then you can sync.
On a different note that may help in other ways, your post model has a big int as its id:
Schema::create('posts', function (Blueprint $table) {
$table->bigIncrements('id');
}
But your post_tag table is only an integer. This mismatch may cause some issues. Suggest changing to match:
Schema::create('post_tag', function (Blueprint $table) {
$table->bigIncrements('id');
$table->integer('post_id')->unsigned(); // <--- suggest change this to bigInteger
$table->foreign('post_id')->references('id')->on('posts');

Answer 2

I may be wrong, but since you don't have the onUpdate or onDelete properties in your table associated with your foreign keys, it could be the problem.

Answer 3

The correct solution was this:
$post = auth()->user()->posts()->create([
'caption' => $data['caption'],
'image' => $imagePath,
]);
I just added this to my code, and all was ok.