[{"category":[{"id":ObjectId("60ed3d041436f1310401b1b2"),"name":"cat 1","services":[{"_id":ObjectId("607ab64bc520c2587045694b"),"name":"service 1",}]}]},{"category":[{"id":ObjectId("60ed3cff1436f1310401b1b1"),"name":"cat 2","services":[{"_id":ObjectId("5ffd5d4bc4967a1b10bf5022"),"name":"service 2",}]},{"id":ObjectId("60ed3cff1436f1310401b1b1"),"name":"cat 2","services":[{"_id":ObjectId("6077e777ad481470d42c4e41"),"name":"service 3",}]}]}]

db.collection.aggregate([{$unwind:"$category"},{$group:{_id:{_id:"$_id",name:"$category.name"},services:{$push:{$first:"$category.services"}}}},{$project:{_id:"$_id._id",name:"$_id.name",services:1}}])