Force collection name

@@ -9,6 +9,8 @@ const ChallengeSchema=new Schema({
     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
     isOpen: { type: Boolean }
+},{
+    collection: 'challenge'
 });
 
 ChallengeSchema.statics.findByChallengeName=function(challengeName){

@@ -4,6 +4,8 @@ const { Schema } = mongoose;
 
 const GroupSchema = new Schema({
     members: [{ type: Schema.Types.ObjectId, ref: 'User' }]
+},{
+    collection: 'group'
 });
 
 const Group = mongoose.model('Group',GroupSchema);

@@ -5,6 +5,8 @@ const { Schema } = mongoose;
 const ParticipationSchema = new Schema({
     sessionId: { type: Schema.Types.ObjectId, ref: 'Session' },
     groupId: { type: Schema.Types.ObjectId, ref: 'Group' }
+},{
+    collection: 'particiaption'
 });
 
 const Participation = mongoose.model('Participation', ParticipationSchema);

@@ -10,6 +10,8 @@ const ProblemSchema=new Schema({
     correctNum: {type: Number, required: true},
     count: { type: Number },
     category: {type:[String]}
+},{
+    collection: 'problem'
 });
 
 ProblemSchema.statics.findByProblemNum=function(problemNum){
@@ -59,5 +61,5 @@ ProblemSchema.methods.serialize=function(){
     return this.toJSON();
 }
 
-const Problem=mongoose.model('Problem',ProblemSchema);
-module.exports=Problem;
\ No newline at end of file
+const Problem = mongoose.model('Problem',ProblemSchema);
+module.exports = Problem;
\ No newline at end of file

@@ -5,7 +5,10 @@ const { Schema } = mongoose;
 const SessionSchema = new Schema({
     challengeId: { type: Schema.Types.ObjectId, ref: 'Challenge' },
     sessionStartDate: { type: Object },
-    sessionEndDate: { type: Object }
+    sessionEndDate: { type: Object },
+    isOpen: { type: Boolean  }
+},{
+    collection: 'session'
 });
 
 const Session = mongoose.model('Session', SessionSchema);

@@ -13,6 +13,8 @@ const UserSchema = new Schema({
   friendList: [{ type: Schema.Types.ObjectId, ref: 'User' }],
   slackWebHookURL: String,
   goalNum: Number,
+},{
+  collection: 'user'
 });
 
 UserSchema.statics.findByUsername = function (username) {