Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (Laravel Application)

Question

I'm having this error in my laravel application.

these are the tables:

Post

Schema::create('posts', function (Blueprint $table) {
        $table->increments('id');
        $table->string('title');
        $table->text('content');
        $table->timestamps();
        $table->integer('user_id')->unsigned();
});

Categories

Schema::create('categories', function (Blueprint $table) {
        $table->increments('id');
        $table->string('name');
        $table->timestamps();
});

Post_Category

Schema::create('post_category', function (Blueprint $table) {
       $table->integer('post_id')->unsigned()->unique()->nullable();
       $table->integer('cat_id')->unsigned()->unique()->nullable();
       $table->timestamps();
});

Foreign Keys

Schema::table('posts', function (Blueprint $table) {
       $table->foreign('id')->references('post_id')->on('post_category')->onDelete('cascade');
});

Schema::table('categories', function (Blueprint $table) {
      $table->foreign('id')->references('cat_id')->on('post_category')->onDelete('cascade');
});

Here my models

class Categorie extends Model
{
    protected $table = 'categories';

    public function posts() {
      return $this->hasMany('App\PostCategory');
    }
}

...

class Post extends Model
{
    public function author() {
      return $this->belongsTo('App\User', 'user_id');
    }

    public function featuredImage() {
      return $this->belongsTo('App\Media', 'media_id');
    }

    public function categories() {
      return $this->hasMany('App\PostCategory');
    }
}

...

class PostCategory extends Model
{
  protected $table = 'post_category';
}

Here the controllers

Store Category:

public function store(StoreCategory $request)
    {
        $category = new Categorie;
        $category->name = $request->input('name');
        $category->save();

        return redirect('/admin/categories')->with('status', 'New category created!');
    }

Store post

public function store(StorePost $request)
    {
        $post = new Post;
        $post->title = $request->input('title');
        $post->content = $request->input('content');
        $post->media_id = $request->input('media_id');
        $post->user_id = $request->input('user_id');
        $post->save();

        return redirect('/admin/posts')->with('status', 'New post created!');
    }

Having this error

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (the_film_corner.tfc_categories, CONSTRAINT categories_id_foreign FOREIGN KEY (id) REFERENCES tfc_post_category (cat_id) ON DELETE CASCADE) (SQL: insert into tfc_categories (name, updated_at, created_at) values (News, 2017-02-26 14:51:15, 2017-02-26 14:51:15))

Thanks

Samuele Colombo Samuele Colombo · Accepted Answer · 2017-02-26T15:49:07

You add the foreign keys in the wrong table:

Schema::table('post_category', function(Blueprint $table) {
    $table->foreign('post_id')->references('id')->on('post')->onDelete('cascade');
    $table->foreign('category_id')->references('id')->on('category')->onDelete('cascade');
});

Then...

class Categorie extends Model
{
    protected $table = 'categories';

    public function posts() {
      return $this->belongsToMany('App\Category');

     // Or more specifically
     return $this->belongsToMany('App\Category', 'post_category', 'id', 'post_id');
    }
}

class Post extends Model
{
    // ...

    public function categories() {
      return $this->belongsToMany('App\Category');

      // Or more specifically
      return $this->belongsToMany('App\Category', 'post_category', 'id', 'category_id');
    }
}

You can find more information following the official guide for Laravel:

https://laravel.com/docs/5.4/eloquent-relationships#one-to-many

Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (Laravel Application)

2 Answers