throw new InvalidArgumentException('$pipeline is empty');

[719media]

why is an exception thrown for an empty pipeline? Like db.test.aggregate([])?

db.test.aggregate([]) works fine directly in mongo

See https://github.com/mongodb/mongo-php-library/blob/master/src/Operation/Aggregate.php#L115

[719media]

Sorry, one more addition, in the console:
db.test.aggregate()
is also valid (no pipeline), so perhaps the PHP method should have default array parameter:
public function __construct($databaseName, $collectionName, array $pipeline = [], array $options = [])
as well to support this "no pipeline" behavior.

Thanks

[jmikola]

We'll take a look at this and contemplate a change for 1.2.0. I had some mental recollection that this was due to earlier versions of the CRUD specification, which also prohibits bulkWrite() from being called with no operations; however, I wasn't able to find specific mention of the aggregate pipeline. The non-empty pipeline requirement is also inconsistent across various language drivers (PHP isn't the only one with this behavior, but many others do allow empty pipelines).

[jmikola]

Tracking in PHPLIB-281.