[{"description":"PC-13","comments":"","parts":[{"id":"PARTS-23aq","type":"mouse","description":""},{"id":"PARTS-18be","type":"keyboard","description":""},{"id":"PARTS-53ih","type":"monitor","description":""}]},{"description":"PC-09","comments":"","parts":[{"id":"PARTS-31me","type":"monitor","description":""},{"id":"PARTS-36hs","type":"mouse","description":""},{"id":"PARTS-74bc","type":"keyboard","description":""}]},{"description":"PC-49","comments":"","parts":[{"id":"PARTS-48up","type":"monitor","description":""},{"id":"PARTS-90hz","type":"mouse","description":""},{"id":"PARTS-14zg","type":"keyboard","description":""}]}]

db.collection.aggregate([{$group:{_id:null,allItems:{$push:"$$ROOT"}}}])