[{"_id":"1234.abc.test1","some_key":"some_value"},{"_id":"1234.abc.test2","some_key":"some_value"},{"_id":"0420.def.test3","some_key":"some_value"}]

db.collection.aggregate([{$project:{first:{"$arrayElemAt":[{"$split":["$_id","."]},0]}}},{"$group":{"_id":"$first"}}])