Laravel无法添加或更新子行：外键约束失败

Question

Before you start giving vote-down or mark as duplicated please read:

I have read Cannot add or update a child row a foreign key constraint fails and it is not my case,

1. I have existing record
2. my table supports cascade on delete.
3. I also have my unsigned columns nullable

-- codes provided below

Issue

I am trying save related data in 3rd table named product_attributes with sync method, which gets product_id and attribute_id

Code

schema

public function up()
{
        Schema::create('product_attributes', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('product_id')->nullable()->unsigned();
            $table->integer('attribute_id')->nullable()->unsigned();
        });
        Schema::table('product_attributes', function (Blueprint $table) {
            $table->foreign('product_id')->references('id')->on('products')->onDelete('cascade');
            $table->foreign('attribute_id')->references('id')->on('attributes')->onDelete('cascade');
        });
}

product model

public function attributes(){
        return $this->belongsToMany(Attribute::class, 'product_attributes', 'product_id', 'attribute_id');
}

attribute model

public function products(){
        return $this->belongsToMany(Product::class);
}

product controller

$product->save()
$product->attributes()->sync($request->attributes, false);

blade

<select class="form-control" name="attributes[]" id="attributes" multiple="multiple">
  @foreach($attributes as $attribute)
    <option value="{{ $attribute->id }}">{{ $attribute->title }}</option>
  @endforeach
</select>

Error

this is what I'm getting:

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`shopping`.`product_attributes`, CONSTRAINT `product_attributes_attribute_id_foreign` FOREIGN KEY (`attribute_id`) REFERENCES `attributes` (`id`) ON DELETE CASCADE) (SQL: insert into `product_attributes` (`attribute_id`, `product_id`) values (*parameters, 3))

Anyone can help with that?

Update

Here is what I get when I send my data to controller