Integrity constraint violation: 1452 Cannot add or update a child row:

It just simply means that the value for column project_id on table comments you are inserting doesn't exist on table projects. Bear in mind that the values of column project_id on table comments is dependent on the values of ID on table Projects.

The value 50dc845a-83e4-4db3-8705-5432ae7aaee3 you are inserting for column project_id does not exist on table projects.

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Make sure you have project_id in the fillable property of your Comment model.

I had the same issue, And this was the reason.

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If you are adding new foreign key to an existing table and the columns are not null and not assigned default value, you will get this error,

Either you need to make it nullable or assign default value, or delete all the existing records to solve it.

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Also make sure that the foreign key you add is the same type of the original column, if the column you're reference is not the same type it will fail too.

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I hope my decision will help. I had a similar error in Laravel. I added a foreign key to the wrong table. Wrong code:

Schema::create('comments', function (Blueprint $table) {
$table->unsignedBigInteger('post_id')->index()->nullable();
...
$table->foreign('post_id')->references('id')->on('comments')->onDelete('cascade');
    });


Schema::create('posts', function (Blueprint $table) {
    $table->bigIncrements('id');
    ...
    });

Please note to the function on('comments') above. Correct code

 $table->foreign('post_id')->references('id')->on('posts')->onDelete('cascade');

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that means that the value for column project_id on table comments you are inserting not only doesn't exist on table projects BUT also project_id probably doesn't have default value. E.g. in my case I set it as NULL.

As for Laravel you can consider this expressions as a chunk of code of a migration php file, for example:

class ForeinToPhotosFromUsers extends Migration

{ /** * Run the migrations. * * @return void */

public function up()
{
    Schema::table('users', function (Blueprint $table) {
    $table->unsignedBigInteger('photo_id')->nullable();// ! ! ! THIS STRING ! ! !
    $table->foreign('photo_id')->references('id')->on('photos');
});
}

}

Obviously you had to create the Model class(in my case it was Photo) next to all these.

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First delete the constraint "fk_comments_projects1" and also its index. After that recreate it.

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You also get this error if you do not create and populate your tables in the right order. For example, according to your schema, Comments table needs user_id, project_id, task_id and data_type_id. This means that Users table, Projects table, Task table and Data_Type table must already have exited and have values in them before you can reference their ids or any other column.

In Laravel this would mean calling your database seeders in the right order:

class DatabaseSeeder extends Seeder
{
    /**
     * Seed the application's database.
     *
     * @return void
     */
    public function run()
    {
        $this->call(UserSeeder::class);
        $this->call(ProjectSeeder::class);
        $this->call(TaskSeeder::class);
        $this->call(DataTypeSeeder::class);
        $this->call(CommentSeeder::class);
    }
}

This was how I solved a similar issue.

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Maybe you have some rows in the table that you want to create de FK.

Run the migration with foreign_key_checks OFF Insert only those records that have corresponding id field in contents table.

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Issue could be because of no project record found in database with project_id you are trying to add...

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In case someone is using Laravel and is getting this problem. I was getting this as well and the issue was in the order in which I was inserting the ids (i.e., the foreign keys) in the pivot table.

To be concrete, find below an example for a many to many relationship:

wordtokens <-> wordtoken_wordchunk <-> wordchunks

// wordtoken_wordchunk table
Schema::create('wordtoken_wordchunk', function(Blueprint $table) {
        $table->integer('wordtoken_id')->unsigned();
        $table->integer('wordchunk_id')->unsigned();

        $table->foreign('wordtoken_id')->references('id')->on('word_tokens')->onDelete('cascade');
        $table->foreign('wordchunk_id')->references('id')->on('wordchunks')->onDelete('cascade');

        $table->primary(['wordtoken_id', 'wordchunk_id']);
    });

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

// wordtokens table
Schema::create('word_tokens', function (Blueprint $table) {
        $table->increments('id');
        $table->string('text');
});

Now my models look like follows:

class WordToken extends Model
{
   public function wordchunks() {
      return $this->belongsToMany('App\Wordchunk');
   }
}

class Wordchunk extends Model
{

    public function wordTokens() {
        return $this->belongsToMany('App\WordToken', 'wordtoken_wordchunk', 'wordchunk_id', 'wordtoken_id');
    }
}

I fixed the problem by exchanging the order of 'wordchunk_id' and 'wordtoken_id' in the Wordchunk model.

For code completion, this is how I persist the models:

private function persistChunks($chunks) {
    foreach ($chunks as $chunk) {
        $model = new Wordchunk();
        $model->text = implode(' ', array_map(function($token) {return $token->text;}, $chunk));
        $tokenIds = array_map(function($token) {return $token->id;}, $chunk);
        $model->save();
        $model->wordTokens()->attach($tokenIds);
    }
}

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I had this issue when I was accidentally using the WRONG "uuid" in my child record. When that happens the constraint looks from the child to the parent record to ensure that the link is correct. I was generating it manually, when I had already rigged my Model to do it automatically. So my fix was:

$parent = Parent:create($recData); // asssigning autogenerated uuid into $parent

Then when I called my child class to insert children, I passed this var value:

$parent->uuid

Hope that helps.

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I had the same error, the problem was that I was trying to add role_id foreign to the users table, but role_id did not have a default value, so the DB did not allow me to insert the column because I already had some users and it didn't know how to add the column to them. Since I was in development I just used migrate:fresh, but if I was on production, I would probably set a default value to the role_id and not make it not constrained until I had the corresponding role on the DB.

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Could be you are inserting a foreign key value that does not match with that of the primary key.

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In my case it didn't work because I was leaving a space in one of the names of in the model.

https://i.stack.imgur.com/tAIW8.jpg

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I had a table fulfilled with some entries that has dead foreigners keys. once i cleaned the table, i was able to add a new constraint.

DELETE FROM `table_with_bad_entries` WHERE `expected_foreign_key` not in (select `id` from `the_referenced_table`);

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I just exported the table deleted and then imported it again and it worked for me. This was because i deleted the parent table(users) and then recreated it and child table(likes) has the foreign key to parent table(users).

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