[{"user":"abc","sessions":{"12345":{"pages":[{requests:[{name:"request1"}]}]},"23456":{"pages":[{requests:[{name:"request1"}]}]}}},{"user":"xyz","sessions":{"12345":{"pages":[{requests:[{name:"request1"}]}]},"23456":{"pages":[{requests:[{name:"request1"}]}]}}}]

db.collection.aggregate([{$project:{pages:{$reduce:{input:{$objectToArray:"$sessions"},initialValue:[],in:{$concatArrays:["$$value",{$reduce:{input:"$$this.v.pages",initialValue:[],in:{$concatArrays:["$$value","$$this.requests"]}}}]}}}}},{$group:{_id:null,pages:{$push:"$pages"}}},{$project:{pages:{$reduce:{input:"$pages",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}}])