PHP Activerecord - Join on the same table

Question

I'm using php.activerecord and now I'm trying to to a join on the same table but somehow it doesn't work.

I want to achieve something like this:
http://sqllessons.com/categories.html

I have three main categories and n x subcategories. What I want to do is, to join this table (like in the example link) and achieve something like this:

ID     |     Category     |     is a Subcategory of
1      | Main 1           | NULL
2      | Main 2           | NULL
3      | Main 3           | NULL
4      | Sub 1            | Main 1
5      | Sub 2            | Main 2
6      | Sub 3            | Main 1

I've tried a simple join on the same table through the model

 static $has_many = array(
    array('category')
 );

and

$data = Category::all(array('joins' => array('category')));  

but I just get this error:

Fatal error: Uncaught exception 'ActiveRecord\DatabaseException' with message 'exception 'PDOException' with message 'SQLSTATE[42000]: Syntax error or access violation: 1066 Not unique table/alias: 'rezepte_category'' in /home/www/xxxxx/xxxxx/classes/activerecord/lib/Connection.php on line 325    

Anyone an idea?

UPDATE 1:
Apparently I have to set an ALIAS. I did this:

    static $has_many = array(
    array('subcategory', 'class'=>'category','foreign_key' => 'id')
);  

But still the same error.

Answer 1

I think you also need a

static $belongs_to = array(array('category'));

in your model. If that does not help, you may want to try a custom sql join:

$join = 'LEFT JOIN category c ON(category.category_id = c.parent_id)';
$categories = Category::all(array('joins' => $join));

The documentation will provide further information: http://www.phpactiverecord.org/projects/main/wiki/Finders#joins

Answer 2

If you define a relation you have to consider 2 keys: the foreign key, and the primary key. In the case of a parent->child relation you have 2 relations as well: a child has a parent, and the other way around, a parent can have children.

The first, your belongs_to, would have as foreign key the child's id, and as primary key the id of the parent. (this looks wrong on your update by the way!).
But the second, the one that says parent has multiple children, ALSO has as primary key the id field, because that is the primary key, the 'source' of this information. Don't try to mix those up because you'd have a "one way" or "the other way" relation, because there is only one relation here, but you're defining it twice :)

So make sure you set both your foreign and your primary keys correct.