LunaQL docs are incomplete.
Reference
clauses
groupBy

groupBy

The groupBy clause is used to group documents by a specific property:

posts.lunaql
query({
	from({
		posts: {
			groupBy(["user_id"]);
		};
	});
});

You can also group by multiple properties:

posts.lunaql
query({
	from({
		posts: {
			groupBy(["user_id", "created_at"]);
		};
	});
});
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When using groupBy, LunaQL will add a _count property to all the grouped objects.