[{_id:"5df103deb89c662a18c77b04",created:1576076254893,updated:1576076254893,name:"Test category",parents:[{_id:"5df104c50620032a27f9bce8",created:1576076485495,updated:1576076485495,name:"Parent Parent Category 2",parents:[],tags:[],defaultIndex:5},{_id:"5df104c50620032a27f9bce9",created:1576076485495,updated:1576076485495,name:"Parent Parent Category",parents:[],tags:[],defaultIndex:5},{_id:"5df104c50620032a27f9bcea",created:1576076485495,updated:1576076485495,name:"Parent Category",parents:["5df104c50620032a27f9bce8","5df104c50620032a27f9bce9"],tags:[],defaultIndex:5}],tags:[],defaultIndex:5},{_id:"5df103deb89c662a18c77b05",created:1576076254893,updated:1576076254893,name:"Test category 2",parents:[],tags:[],defaultIndex:5}]

db.collection.aggregate([{$project:{_id:0,toUnwind:{$concatArrays:["$parents",[{"$mergeObjects":["$$ROOT",{parents:[]}]}]]}}},{$unwind:"$toUnwind"},{$replaceRoot:{newRoot:"$toUnwind"}}])