How to find into mongodb to the last item of an array?

Question

I want to find documents where last elements in an array equals to some value. Array elements may be accessed by specific array position:

// i.e. comments[0]         


        

Answer 1

You can't do this in one go with this schema design. You can either store the length and do two queries, or store the last comment additionally in another field:

{
    '_id': 'foo';
    'comments' [
        { 'value': 'comment #1', 'by': 'Ford' },
        { 'value': 'comment #2', 'by': 'Arthur' },
        { 'value': 'comment #3', 'by': 'Zaphod' }
    ],
    'last_comment': {
        'value': 'comment #3', 'by': 'Zaphod'
    }
}

Sure, you'll be duplicating some data, but atleast you can set this data with $set together with the $push for the comment.

$comment = array(
    'value' => 'comment #3',
    'by' => 'Zaphod',
);

$collection->update(
    array( '_id' => 'foo' ),
    array(
        '$set' => array( 'last_comment' => $comment ),
        '$push' => array( 'comments' => $comment )
    )
);

Finding the last one is easy now!