Calculated Fields

You can use accessors. In Eloquent it's very simple - you can just add methods named "get[Name]Attribute", where [Name] equals to the name of the calculated attribute.

For example, if you store user's bithday in the database, you can add an accessor to get his age:

protected $dates = ['birthday']; // this tells Eloquent to cast "birthday" attribute into a Carbon instance

public function getAgeAttribute() {
    return $this->birthday->age;
}

Now you can use it as a normal attribute:

echo "User's age: " . $user->age;

What if the calculation is expensive and you want to cache it, but not persist it? You obviously don't need your calculated fields in the database. What are the options?

@blonkm

So use Cache! Laravel implements many cache drivers that are very easy to use like: Cache::put ('model.434.age', $calculated_age)

Also you have Cache::remember () that allow you passing a callback to define the cache value if not cache yet.

Take a look at docs!

Can this calculated attribute be used is queries. Can I write something like User::where('age', ' > ', 100)

You could do this if you use the methods on the collection. https://laravel.com/docs/5.3/eloquent-collections#available-methods

for example; `$collection = collect([ ['product' => 'Desk', 'price' => 200], ['product' => 'Chair', 'price' => 100], ['product' => 'Bookcase', 'price' => 150], ['product' => 'Door', 'price' => 100], ]);

$filtered = $collection->where('price', 100);

$filtered->all();

/* [ ['product' => 'Chair', 'price' => 100], ['product' => 'Door', 'price' => 100], ] */`

You can specify your calculated field in your select as follows:

$model->select(['*', \DB::raw('(initialAmount+interest-(writeOffs+credits+paid+leftOver)) AS balance')]);