Database
[{line:"car",number:"1",category:{FERRARI:{color:"blue"},LAMBORGHINI:{color:"red"}}},{line:"car",number:"2",category:{FERRARI:{color:"blue"}}}]
Query
db.collection.aggregate([{$match:{$and:[{line:"car"},{number:{$in:["1","2"]}}]}},{"$addFields":{"category.LAMBORGHINI":{$cond:[{"$ifNull":["$category.LAMBORGHINI",false]},{$mergeObjects:["$category.LAMBORGHINI",{number:"$number"}]},"$$REMOVE"]}}},{$group:{_id:null,CATEGORIES:{$addToSet:"$category.LAMBORGHINI"}}}])