Add participation, session and group schema and Edit challenge and user schema

 const mongoose = require("mongoose");
 
-const { Schema }=mongoose;
-
-const GroupSchema=new Schema({
-    members:{type:[String]},
-});
+const { Schema } = mongoose;
 
 const ChallengeSchema=new Schema({
     challengeName: {type: String, required: true},
@@ -12,24 +8,13 @@ const ChallengeSchema=new Schema({
     endDate: {type: Object, required: true},
     durationPerSession: {type: String, required: true}, // '1d' means one day per session, '2w' means 2 weeks per session, '3m' means 3 months per session.
     goalPerSession: {type: Number, required:true}, // number of problems for one session
-    groups:{type: [GroupSchema], required:true}, // groups attending challenge, group of only one member supposed to be single
+    isOpen: { type: Boolean }
 });
 
 ChallengeSchema.statics.findByChallengeName=function(challengeName){
     return this.findOne({challengeName:challengeName});
 }
 
-ChallengeSchema.methods.addNewGroup=function(group){
-    this.groups.push(group);
-    return this.save();
-}
-
-ChallengeSchema.methods.removeGroup=function(group_id){
-    const idx=this.groups.findIndex((item)=>item._id===group_id);
-    this.groups.splice(idx,1);
-    return this.save();
-}
-
 ChallengeSchema.methods.getChallengeName=function(){
     return this.challengeName;
 }
@@ -50,13 +35,9 @@ ChallengeSchema.methods.getGoalPerSession=function(){
     return this.goalPerSession;
 }
 
-ChallengeSchema.methods.getGroups=function(){
-    return this.groups;
-}
-
 ChallengeSchema.methods.serialize=function(){
     return this.toJSON();
 }
 
-const Challenge=mongoose.model('Challenge',ChallengeSchema);
-module.exports=Challenge;
\ No newline at end of file
+const Challenge = mongoose.model('Challenge', ChallengeSchema);
+module.exports = Challenge;
\ No newline at end of file

+const mongoose = require("mongoose");
+
+const { Schema } = mongoose;
+
+const GroupSchema = new Schema({
+    members: [{ type: Schema.Types.ObjectId, ref: 'User' }]
+});
+
+const Group = mongoose.model('Group',GroupSchema);
+module.exports = Group;
\ No newline at end of file

+const mongoose = require("mongoose");
+
+const { Schema } = mongoose;
+
+const ParticipationSchema = new Schema({
+    sessionId: { type: Schema.Types.ObjectId, ref: 'Session' },
+    groupId: { type: Schema.Types.ObjectId, ref: 'Group' }
+});
+
+const Participation = mongoose.model('Participation', ParticipationSchema);
+module.exports = Participation;
\ No newline at end of file

+const mongoose = require("mongoose");
+
+const { Schema } = mongoose;
+
+const SessionSchema = new Schema({
+    challengeId: { type: Schema.Types.ObjectId, ref: 'Challenge' },
+    sessionStartDate: { type: Object },
+    sessionEndDate: { type: Object }
+});
+
+const Session = mongoose.model('Session', SessionSchema);
+module.exports = Session;
\ No newline at end of file

@@ -10,7 +10,7 @@ const UserSchema = new Schema({
   userBJID: String,
   sovledBJ: Object,
   solvedBJ_date: Object,
-  friendList: [String],
+  friendList: [{ type: Schema.Types.ObjectId, ref: 'User' }],
   slackWebHookURL: String,
   goalNum: Number,
 });