db={"testA":[{"_id":ObjectId("60508eeb52a2ab598a013bb9"),"message":"hello","field1":false,"field2":1,"field3":"something"},{"_id":ObjectId("60508f2852a2ab598a013bba"),"message":"hello","field1":false,"field2":1,"field3":"something"},{"_id":ObjectId("60508f2852a2ab665a013bba"),"message":"hello","field1":false,"field2":1,"field3":"something"}],"testB":[{"_id":ObjectId("60508f4152a2ab598a013bbb"),"aId":ObjectId("60508f2852a2ab598a013bba")},{"_id":ObjectId("60508f4552a2ab598a013bbc"),"aId":ObjectId("60508f2852a2ab598a013bba")},{"_id":ObjectId("605090a452a2ab598a013bbd"),"aId":ObjectId("60508eeb52a2ab598a013bb9")}]}

db.testA.aggregate([{"$lookup":{"from":"testB","localField":"_id","foreignField":"aId","as":"join"}},{$project:{count:{$size:"$join"}}},{$set:{favorite:{$switch:{branches:[{case:{$gte:["$count",1]},then:true},{case:{$gte:["$count",0]},then:false},],default:false}}}}])