Question

One thing with which I have long had problems, within the CakePHP framework, is defining simultaneous hasOne and hasMany relationships between two models. For example:

BlogEntry hasMany Comment
BlogEntry hasOne MostRecentComment (where MostRecentComment is the Comment with the most recent created field)

Defining these relationships in the BlogEntry model properties is problematic. CakePHP’s ORM implements a has-one relationship as an INNER JOIN, so as soon as there is more than one Comment, BlogEntry::find('all') calls return multiple results per BlogEntry.

I’ve worked around these situations in the past in a few ways:

1. Using a model callback (or, sometimes, even in the controller or view!), I’ve simulated a MostRecentComment with:
   $this->data['MostRecentComment'] = $this->data['Comment'][0];
   This gets ugly fast if, say, I need to order the Comments any way other than by Comment.created. It also doesn’t Cake’s in-built pagination features to sort by MostRecentComment fields (e.g. sort BlogEntry results reverse-chronologically by MostRecentComment.created.

2. Maintaining an additional foreign key, BlogEntry.most_recent_comment_id. This is annoying to maintain, and breaks Cake’s ORM: the implication is BlogEntry belongsTo MostRecentComment. It works, but just looks…wrong.

These solutions left much to be desired, so I sat down with this problem the other day, and worked on a better solution. I’ve posted my eventual solution below, but I’d be thrilled (and maybe just a little mortified) to discover there is some mind-blowingly simple solution that has escaped me this whole time. Or any other solution that meets my criteria:

• it must be able to sort by MostRecentComment fields at the Model::find level (ie. not just a massage of the results);
• it shouldn’t require additional fields in the comments or blog_entries tables;
• it should respect the ‘spirit’ of the CakePHP ORM.

(I’m also not sure the title of this question is as concise/informative as it could be.)

Answer 1

The solution I developed is the following:

class BlogEntry extends AppModel
{
    var $hasMany = array( 'Comment' );

    function beforeFind( $queryData )
    {
        $this->_bindMostRecentComment();

        return $queryData;
    }

    function _bindMostRecentComment()
    {
        if ( isset($this->hasOne['MostRecentComment'])) { return; }

        $dbo = $this->Comment->getDatasource();
        $subQuery = String::insert("`MostRecentComment`.`id` = (:q)", array(
            'q'=>$dbo->buildStatement(array(
                'fields' => array( String::insert(':sqInnerComment:eq.:sqid:eq', array('sq'=>$dbo->startQuote, 'eq'=>$dbo->endQuote))),
                'table'  => $dbo->fullTableName($this->Comment),
                'alias'  => 'InnerComment',
                'limit'  => 1,
                'order'  => array('InnerComment.created'=>'DESC'),
                'group'  => null,
                'conditions' => array(
                    'InnerComment.blog_entry_id = BlogEntry.id'
                )
            ), $this->Comment)
        ));

        $this->bindModel(array('hasOne'=>array(
            'MostRecentComment'=>array(
                'className' => 'Comment',
                'conditions' => array( $subQuery )
            )
        )),false);

        return;
    }

    // Other model stuff
}

The notion is simple. The _bindMostRecentComment method defines a fairly standard has-one association, using a sub-query in the association conditions to ensure only the most-recent Comment is joined to BlogEntry queries. The method itself is invoked just before any Model::find() calls, the MostRecentComment of each BlogEntry can be filtered or sorted against.

I realise it’s possible to define this association in the hasOne class member, but I’d have to write a bunch of raw SQL, which gives me pause.

I’d have preferred to call _bindMostRecentComment from the BlogEntry’s constructor, but the Model::bindModel() param that (per the documentation) makes a binding permanent doesn’t appear to work, so the binding has to be done in the beforeFind callback.