Laravel Model Define Parameters Won't Insert - laravel

Question

I am currently moving over from symfony to laravel, it's quite a bit different when it comes to the database. So i have a basic model, i'm just going to use an example:
class Test extends Model
{
use HasFactory;
}
All good, i have a migration and the table created. However, i don't like this:
$test = new Test();
$test->my_field = 'hello';
$test->save();
I don't like it because it's having to use a magic __set() to create the parameter, if i define the parameter in my model like this:
class Test extends Model
{
use HasFactory;
public ?string $my_field;
}
I get database errors when it tries to insert when i define the params like this. Why is that? It's doing the same thing as __set() but i'm actually physically defining them, which in my opinion is a better way to code it as my IDE can typehint and it's just nicer to follow the program knowing what params are there.
What's the reason for it inserting when i don't define them, and not when i do? From my actual table which is bookings , has a field booking_ref:
General error: 1364 Field 'booking_ref' doesn't have a default value (SQL: insert into booking_reviews (updated_at, created_at) values (2021-12-13 14:13:08, 2021-12-13 14:13:08))
This happens when i define the $booking_ref param on the model, but if i take it out and rely on the __set() method it works fine. Doesn't make any sense to me right now.

Answer 1

I think this is a reasonable enough misunderstanding to be useful to future visitors, so I want to try to explain what's going on with some pseudo-code and some references to the current source code.
You are correct that when setting a property on a Laravel model, that is a column in the DB, internally Laravel is using the PHP magic method __set.
What this does is allow you to 1) set properties directly instead of calling some kind of setter function, and 2) interact with your table columns without needing the boilerplate of column definitions in your model.
Where the assumptions go wrong is with what __set is doing. __set does not have to simply set an actual property with the same name. __set is just a method you may implement to do whatever you want. What you assumption implies is that it's doing something like this:
public function __set($key, $value)
{
$this->{$key} = $value;
}
However, you can do whatever you want with the $key and $value passed to the magic method.
What Laravel does is call another method defined in the HasAttributes trait - setAttribute.
public function __set($key, $value)
{
$this->setAttribute($key, $value);
}
setAttribute does a few extra things, but most importantly it adds the key/value pair to Model property $this->attributes[].
To hopefully help this difference make sense, here is what the two __set methods would yield with a basic example:
$model->my_column = 'value';
// 1st example
/**
* {
* public $my_column = 'value';
* }
*/
// Laravel way
/**
* {
* protected $attributes= ['my_column => 'value'];
* }
*/
I won't go through both saving and updating since they're very similar, but to show how this is used, we can look at the save method, which calls performInsert and after a few more calls makes it's way back to the attributes property to determine what to actually insert into the query.
Summary
Laravel does not use custom model properties when deciding what column/values to add to queries.
This is why when you create custom mutators, you interact with the attributes property just like Laravel does internally.
Anytime you introduce "magic" into code, you have some tradeoffs. In this case, that tradeoff is slightly less clarity with what database columns are actually available. However, like I mentioned in comments, there are other solutions to make models more IDE friendly like Laravel IDE helper.