PHPLIB-417: countDocuments should use group with _id: 1

[alcaeus]

https://jira.mongodb.org/browse/PHPLIB-417

[lindelius]

Isn't this just going to return either 1 or 0, though?

[alcaeus]

Isn't this just going to return either 1 or 0, though?

The countDocuments helper doesn't rely on collection metadata to return a document count, but rather runs an aggregation pipeline containing a $group pipeline stage to count the documents. In this case, we're grouping by the value 1, which will cause the $group stage to emit a single document with the n field containing the number of documents that were emitted from the previous $match, $skip, and $limit stages.

[lindelius]

Is the ”output a single document” functionality being changed to 1 from null? The shell documentation is still using null in the examples and we’re using the same in quite a few places that’ll have to be changed too if that’s the case

[alcaeus]

The motivation for the change is explained in mongodb/specifications#504:

The $group stage of an aggregation pipeline can use any constant to achieve the group-for-all effect. Switching to using 1 as a constant, to avoid test runner issues associated with null values

This has no implications on the result or on whether you should use null or 1 when doing such a $group aggregation yourself. In our case the change is necessary to pass the spec tests once adding support for retryable reads.

The "output a single document" functionality is not tied to anything: this is a side effect of using $group with a constant value for _id, no matter what this constant value is. You might as well use _id: 'The quick brown fox jumps over the lazy dog'.

[lindelius]

Ok! Thanks for clarifying. I was under the impression that this was only for when setting the _id field to null.

It’s propbably not your table, but it would be useful to update the documentations to read ”constant value” rather than ”null” for the examples etc.

[alcaeus]

It’s propbably not your table, but it would be useful to update the documentations to read ”constant value” rather than ”null” for the examples etc.

I don't know which table you are referring to. A quick search led me to the documentation for $group, which states:

The _id field is mandatory; however, you can specify an _id value of null, or any other constant value, to calculate accumulated values for all the input documents as a whole.

[kvwalker]

looks good to me!

[jmikola]

@lindelius: Nothing to worry about on your end (or any other application). For purposes of $group, any constant value (provided it's not some special operator/expression) should provide the same effect of grouping into a single accumulation bucket.

The impetus for this change was that several internal specification tests had trouble asserting outgoing commands because those same tests often use null to signify that a field should not exist in the document, rather than asserting that it contains a BSON null value.

It’s propbably not your table, but it would be useful to update the documentations to read ”constant value” rather than ”null” for the examples etc.

@alcaeus linked to the current documentation, but for reference this change was made in mongodb/docs#3542 in the manual for server versions 4.0+. I'm not sure if it was back-ported, but it's possible you were looking at $group documentation from an older version. If there's another instance in the 4.0+ docs that we missed, please let us know, though (or use the edit link in the server manual to submit a PR!).