Add a usable join array model interface.

[romaninsh]

In my example I have table "Nominal" which is hierarchical. When I load it, I create an array like this:

"id" -> "path"

This is handy for be because now that when I'm reading from another model and it contain "id" value, I can look up full nominal path without worrying about number of selects.

I believe Agile Data can do this for me special join type. Here is how I'd like it to work:

$n = new Model_Nominal($db);
$assoc = $n->getPathAssociations();  // returns "id" -> "path"

// Next create a model
$nar = new Model($assoc));

// Test it
$nar->load(183);
echo $nar[$nar->title_field];  // will output path

$m = new Model_Report_Activity($db);
$m->join($nar, 'nominal_id')->addField('nominal_path');

foreach($m as $row) {
    var_dump($row); // includes 'nominal_path' now
}