[{"_id":ObjectId("5b35019563726438989381d3"),"ref":"123","nom":"pc","Revendeurs":[{"revendeur":"Tdiscount","selecteur_prix":".price","hist_prix":[{"date":ISODate("2018-06-28T00:00:00Z"),"prix":200},{"date":ISODate("2018-06-27T00:00:00Z"),"prix":100.8}]}]}]

db.collection.aggregate([{$unwind:"$Revendeurs"},{$group:{_id:null,prixArr:{$push:"$Revendeurs.hist_prix.prix"}}},{$project:{"_id":0,"max":{$reduce:{input:"$prixArr",initialValue:0,in:{$max:"$$this"}}}}}])