Bad Method Call on save()

Post your update method here

you problem is about https://laravel.com/docs/7.x/routing#route-model-binding

Route::patch('something/{post:slug}', 'PostController@update')->name('update');

or set the key in your model

route:

Route::patch('something/{post}', 'PostController@update')->name('update');

model:

public function getRouteKeyName()
{
    return 'slug';
}

and then controller

    public function update(Post $post, Request $request)
    {   
        // $post = Post::find($slug);

        dd($this->validatePost());

the better way is move the rules to From Request, so you don't need $this->validatePost()

    public function update(Post $post, UpdatePostRequest $request)
    { 

and then your rules should be, otherwise, the client able submit a array

'title' => ['required', 'string', 'max:255'],
'body'  => ['required'],
'slug'  => ['required', 'string', 'max:100'],
'tags'  => ['exists:tags,id'],

Route:

Route::resource('posts', PostsController::class)
    ->except('index', 'show')
    ->parameters(['posts' => 'slug'])
    ->middleware('auth');

Model

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;

class Post extends Model
{
    protected $fillable = [
        'title',
        'body',
        'slug',
        'image_url',
        'category_id'
    ];

    protected $guarded = [];

    public function getRouteKeyName()
    {
        return 'slug';
    }

    public function author()
    {
        return $this->belongsTo('App\Models\User', 'user_id');
    }
    
    public function category()
    {
        return $this->belongsTo('App\Models\Category');
    }

    public function tags()
    {
        return $this->belongsToMany('App\Models\Tag')->withTimeStamps();
    }
    
    use HasFactory;
}

Controller:

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Models\Post;
use App\Models\Tag;
use App\Models\Category;
use Illuminate\Pagination\Paginator;

class PostsController extends Controller
{
    /**
     * Display a listing of the resource.
     *
     * @return \Illuminate\Http\Response
     */
    public function index()
    {   
        if (request('tag')) {
        $post = Tag::where('name', request('tag'))
            ->firstOrFail()->posts;
    } else {
        $post = Post::latest()->simplePaginate(13);
        $category = Category::select('image_url')->get();
    }   
        return view('posts.index', [
            'posts' => $post
        ]);
    }

    /**
     * Show the form for creating a new resource.
     *
     * @return \Illuminate\Http\Response
     */
    public function create()
    {   
        $category = Category::all();
        
        return view('posts.create', [
            'tags' => Tag::all(),
            'category_id' => $category
        ]);
    }

    /**
     * Store a newly created resource in storage.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return \Illuminate\Http\Response
     */
    public function store(Request $request)
    {
        $this->validatePost();

        $post = new Post(request(['title', 'body', 'slug', 'image_url', 'category_id']));
    
        $fileExtension = request('image')->getClientOriginalName();
        $fileName = pathInfo($fileExtension, PATHINFO_FILENAME);
        $extension = request('image')->getClientOriginalExtension();
        $newFileName = $fileName . '_' . time() . '.' . $extension;
        $imgPath = request('image')->storeAs('public/img/post_uploads', $newFileName);
    
        $user = auth()->user();
        $post->image_url = $newFileName;
        $post->user_id = $user->id;
    
        $post->save();
        $post->tags()->attach(request('tags'));
    
        return redirect('/posts');
    }

    /**
     * Display the specified resource.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function show(Post $slug)
    {
        $category = Category::get();
        $post = Post::get();
        return view('posts.show', ['post' => $slug, 'category' => $category]);
    }

    /**
     * Show the form for editing the specified resource.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function edit(Post $slug)
    {   
        $post = Post::find($slug);
        $category = Category::all();

        return view('posts.edit', [
            'post' => $slug,
            'tags' => Tag::all(),
            'category_id' => $category
        ]);
    }

    /**
     * Update the specified resource in storage.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function update(Post $slug, Request $request)
    {   
        dd($this->validatePost());

        $post['user_id'] = auth()->id();

        if ($request->has('image')) {
            $fileExtension = request('image')->getClientOriginalName();
            $fileName = pathInfo($fileExtension, PATHINFO_FILENAME);
            $extension = request('image')->getClientOriginalExtension();
            $newFileName = $fileName . '_' . time() . '.' . $extension;
            $imgPath = request('image')->storeAs('public/img/post_uploads', $newFileName);

            $post->image_url = $newFileName;  
            $post->save();
        }
        
        $post->save();

        $post->tags()->attach(request('tags'));

        return redirect('posts/' . $post->slug);
    }

    /**
     * Remove the specified resource from storage.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function destroy($id)
    {
        $post = Post::findOrFail($id);

        $oldImage = public_path() . '/storage/img/post_uploads/' . $post->image_url;
        if (file_exists($oldImage)){
            unlink($oldImage);
        }

        $post->delete();
        return redirect('/dashboard/posts/');
    }

    protected function validatePost() 
    {
        return request()->validate([
            'title' => ['required', 'max:255'],
            'body'  => ['required'],
            'slug'  => ['required', 'max:100'],
            'tags' => ['exists:tags,id'],
            'category_id' => ['required']
        ]);
    }
}

@newbie360 the problem has been fixed but I now receive another exception that has to do with the attachment of tags.. not sure if by only changing names somewhere?

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '1-1' for key 'post_tag_post_id_tag_id_unique' (SQL: insert into post_tag (created_at, post_id, tag_id, updated_at) values (2021-01-19 00:39:54, 1, 1, 2021-01-19 00:39:54))