[{"_id":ObjectId("570557d4094a4514fc1291d6"),"email":"u1@mail.com","user_type":"1","grade":"A1","room_id":ObjectId("580557d4094a4514fc1291d6")},{"_id":ObjectId("570557d4094a4514fc1291d7"),"email":"u2@mail.com","user_type":"2","grade":"A2","room_id":ObjectId("580557d4094a4514fc1291d6")},{"_id":ObjectId("570557d4094a4514fc1291d8"),"email":"u3@mail.com","user_type":"3","grade":"A2","room_id":ObjectId("580557d4094a4514fc1291d6")},{"_id":ObjectId("570557d4094a4514fc1291d9"),"email":"u4@mail.com","user_type":"2","grade":"A2","room_id":ObjectId("580557d4094a4514fc1291d7")},{"_id":ObjectId("570557d4094a4514fc1291e6"),"email":"u5@mail.com","user_type":"3","grade":"A1","room_id":ObjectId("580557d4094a4514fc1291d7")},{"_id":ObjectId("570557d4094a4514fc1291e7"),"email":"u6@mail.com","user_type":"3","grade":"A2","room_id":ObjectId("580557d4094a4514fc1291d7")},{"_id":ObjectId("570557d4094a4514fc1291e8"),"email":"u7@mail.com","user_type":"2","grade":"A1","room_id":ObjectId("580557d4094a4514fc1291d8")},{"_id":ObjectId("570557d4094a4514fc1291e9"),"email":"u8@mail.com","user_type":"3","grade":"A1","room_id":ObjectId("580557d4094a4514fc1291d8")},]

db.collection.aggregate([{$lookup:{from:"collection",localField:"room_id",foreignField:"room_id",as:"roommates"}},{$match:{user_type:"2",grade:"A2","roommates.user_type":"3"}},{$project:{email:1,roommates:{$map:{input:{$filter:{input:"$roommates",cond:{$eq:["$$this.user_type","3"]}}},in:{email:"$$this.email"}}}}}])