Implement models/challenge.js

+const mongoose = require("mongoose");
+
+const { Schema }=mongoose;
+
+const GroupSchema=new Schema({
+    members:{type:[String]},
+});
+
+const ChallengeSchema=new Schema({
+    challengeName: {type: String, required: true},
+    startDate: {type: Object, required: true},
+    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
+});
+
+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;
+}
+
+ChallengeSchema.methods.getStartDate=function(){
+    return this.startDate;
+}
+
+ChallengeSchema.methods.getEndDate=function(){
+    return this.endDate;
+}
+
+ChallengeSchema.method.getDurationPerSession=function(){
+    return this.durationPerSession;
+}
+
+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