var practiceFinishSchema = new Schema({
  "user": ObjectId,
  "eventTime": Number, // time of finish practice
  "serverTime": ISODate, // time of finish practice
  "topic": String,
  "type": String(enum: ["A", "B", "C", "D", "E", "S"]),
  "finishState": String(enum: ["perfect", "imperfect"]),
  "duration": Number, //the duration from start practice to finish
  "practiceLog": [ // not contained in first version
    {"level": String, "problemId": ObjectId, "duration": Number, "correctness": Boolean}
  ]
})