Laravel在模型中的功能

I have a Laravel model with a simple function in it. But for some reason I get this error:

Relationship method must return an object of type Illuminate\Database\Eloquent\Relations\Relation

Here is my Model:

class Dish extends Model
{
    public function sum() {
        return $this->attributes['begin'] + 10;  
    }
}

In my controller I do:

$model->sum();

Anyone knows how I can add the function to my model?

Many thanks in advance!

If the calculation will be performed with the model data, you do not need to use $this->attributes to get the model data, that way it actually makes it a bit more "dirty". the cleanest way it will be as mention in the comments:

public function sumBegin($default = 10)
{
    return $this->begin + $default;
}

that way we take the begin for the current model being called.