Allow filtering by expressions

[yhabteab]

We should allow Expressions as a filter. E.g

$filter = Filter::all();
$filter->add(new Expression('foo'));

[jrauh01]

I also encountered a similar problem. I wanted to create an alias in the model and then filter by the alias.

Model:

public function getColumns(): array
{
    return [
        'username',
        'username_ci' => new Expression('LOWER(username)')
    ];
}

Filtering:

AbcModel::on($db)->filter(Filter::equal('username_ci', strtolower($user)))->first()

Leading to the following error: error evaluating code: SQLSTATE[42703]: Undefined column: 7 ERROR: column abc.username_ci does not exist

To solve this it should be sufficient to use the expression instead of the alias in the where clause.

Regarding the original issue: I also would appreciate being able to filter by expressions directly without the need of creating a column beforehand.