How to convert below MongoDB query into PHP? - php

Question

Below query worked fine in Studio 3T and Robomongo, But I want to convert it into PHP format,
Following is my query,
db.news.aggregate([
{
$match: { "_id" : "1" }
},
{
"$project": {
"comments": {
"$filter": {
"input": "$comments",
"as": "comment",
"cond": { "$eq": [ "$$comment.status", "active" ] }
}
}
}
}, {
"$project": {
"comments": {
"$slice": [
{
"$slice": [
"$comments",
{
"$subtract": [ { "$size": [ "$comments" ] }, 1 ]
}
]
}, -1
]
}
}
}])
I have tried below, But it giving error "Error: localhost:27017: FieldPath field names may not be empty strings."
PHP converted sample:
<?php
$commentsAggregate=array(
array('$match' => array("_id" => "1")),
array('$project' => array(
"comments" => array(
'$filter' => array(
"input" => "$comments",
"as" => "comment",
"cond" => array('$eq' => array( "$$comment.status", 'active'))
)))),
array('$project' => array(
"comments" => array(
'$slice' => array(array(
'$slice' => array("$comments",array('$subtract' => array( array( '$size' => array("$comments")),1)))
), -1)
)))
);
$cursor=$collectionNews->aggregate($commentsAggregate);
Please help me to convert above query.

Answer 1

The error message "FieldPath field names may not be empty strings" originates from the server. Looking at the example PHP code you've provided, I notice that you're inconsistently using single- and double-quoted strings. In particular, these two strings stand out:
"$$comment.status"
"$comment"
PHP is evaluating variable references inside double-quoted strings. Assuming the local scope does not actually have a $comment variable defined, those strings are going to resolve to "$.status" and "", respectively. As evidenced in this script and execution output on 3v4l.org, those examples should at least result in a PHP notice for an undefined variable (my local PHP configuration happens to report this at the "error" level). If you have no record of that error message, I would suggest the following:
Check your error_reporting configuration.
Ideally, you should report everything (E_ALL) in a development environment. Full reporting is also advisable for production, although there you would likely want to disable display_errors (for security) and instead ensure everything is logged properly.
If it turns out the error was logged, look into how it was missed while debugging this issue.
As for fixing the root cause, you should be mindful to use single-quoted strings when writing MongoDB queries/commands in PHP. There is a note about this in the MongoCollection::find() documentation, but it's not something we repeat on every page, as PHP's double-quoted string evaluation is outside the control of the driver.