Restore _ids

@@ -43,7 +43,6 @@ ChallengeSchema.methods.getStatus=function(){
 
 ChallengeSchema.methods.serialize=function(){
     let challengeJSON = this.toJSON();
-    delete challengeJSON._id;
     return challengeJSON;
 }
 

@@ -24,7 +24,6 @@ GroupSchema.methods.getMembers=function(){
 
 GroupSchema.methods.serialize=function(){
     let groupJSON=this.toJSON();
-    delete groupJSON._id;
     return groupJSON;
 }
 

@@ -31,7 +31,6 @@ ParticipationSchema.methods.addProblem=function(problem){
 
 ParticipationSchema.methods.serialize=function(){
     let participationJSON=this.toJSON();
-    delete participationJSON._id;
     return participationJSON;
 }
 

@@ -59,7 +59,6 @@ ProblemSchema.methods.getCategory=function(){
 
 ProblemSchema.methods.serialize=function(){
     let problemJSON=this.toJSON();
-    delete problemJSON._id;
     return problemJSON;
 }
 

@@ -29,7 +29,6 @@ SessionSchema.methods.getStatus=function(){
 
 SessionSchema.methods.serialize=function(){
     let sessionJSON=this.toJSON();
-    delete sessionJSON._id;
     return sessionJSON;
 }
 

@@ -38,7 +38,6 @@ UserSchema.methods.checkPassword = async function (password) {
 UserSchema.methods.serialize = function () {
   const data = this.toJSON();
   delete data.hashedPassword;
-  delete data._id;
   return data;
 };