# Counts the document matching `query`.
	#
	# Returns `-1` if an error occured. See `Sys::last_mongoc_error`.
	#
	# ~~~
	# var client = new MongoClient("mongodb://mongo:27017/")
	# var db_suffix = "NIT_TESTING_ID".environ
	# var db_name = "test_{db_suffix}"
	# var db = client.database(db_name)
	# var col = db.collection("test")
	# var query = new JsonObject
	# query["foo"] = 10
	# assert col.count(query) > 0
	# ~~~
	fun count(query: JsonObject): Int do
		return native.count(query.to_bson.native)
	end