PHP/MongoDB $set use for update an array in a collection - php

Question

I am trying to update() a specific single array in a collection, but while it works fine with $push parameter on a single, specific array, it does not work with a $set parameter.
I don't quite understand logic behind that, because when I use such an example of $pushing the element:
$post_comment = array('$push' =>
array("comments" => array(
"_id" => new MongoId(),
"comment" => htmlspecialchars($_POST['comment']),
"author" => $user->username,
"date" => new MongoDate()
)
)
);
$entries->update(array(
"_id" => $_GET["id"]), $post_comment);
It gives me an array in a MongoDB database which looks more or less like this (with four items pushed in, respectively) :
{
"_id" : "css-clearfix-explained",
"comments" : [
{
"_id" : ObjectId("540cc940af105b19133c9869"),
"comment" : "aaa",
"author" : "maciejsitko",
"date" : ISODate("2014-09-07T21:08:16.215Z")
},
{
"_id" : ObjectId("540cc943af105b19133c986a"),
"comment" : "bbb",
"author" : "maciejsitko",
"date" : ISODate("2014-09-07T21:08:19.542Z")
},
{
"_id" : ObjectId("540cc946af105b19133c986b"),
"comment" : "ccc",
"author" : "maciejsitko",
"date" : ISODate("2014-09-07T21:08:22.968Z")
}
]
}
Which is basically what I want to have, and logically, works fine according to the documentation. But when I try the same with $set as for to edit an individual comment, in the similar fashion as shown:
$edit_comment = array('$set' =>
array("comments" => array(
"_id" => new MongoId($_POST['cmt-id']),
"comment" => htmlspecialchars($_POST['edit-comment']),
"author" => $user->username,
"date" => new MongoDate()
)
)
);
$entries->update(array(
"_id" => $_GET["id"]), $edit_comment);
It outputs four different arrays in place of the previous arrays, to illustrate that, i'll show what happened when I updated first comment "aaa" to "ddd" :
{
"_id" : "css-clearfix-explained",
"comments" : {
"_id" : ObjectId("540cc940af105b19133c9869"),
"comment" : "ddd\r\n ",
"author" : "maciejsitko",
"date" : ISODate("2014-09-07T21:12:10.833Z")
}
}
All the four array elements were pretty much erased and in their place appeared four fields as four independent array elements.
How come? Shouldn't it just work just fine like the example with $push above?

Answer 1

You didn't specify an index within comments. Therefore, $set replaced the array comments with the associated array supplied.
If you want to update a comment, then change your query in the first argument to match a comment by a unique field. Ex, date. In the second argument use a positional $ operator.
Example:
$edit_comment = array('$set' =>
array("comments.$" => array(
"_id" => new MongoId($_POST['cmt-id']),
"comment" => htmlspecialchars($_POST['edit-comment']),
"author" => $user->username,
"date" => new MongoDate()
)
)
);
// this assumes the post date is unique. On second though use something else.
$query = array( "_id" => $_GET["id"], "comments.date" => $_POST['post-date'])
$entries->update( $query, $edit_comment);
Check this out for more info and better explanation:
MongoDB - $set to update or push Array element