Fix query using _ids

@@ -9,9 +9,17 @@ const GroupSchema = new Schema({
 });
 
 GroupSchema.methods.addGroupMemeber=function(user){
-    this.members.push({$oid:user._id.$oid});
+    this.members.push(user._id);
     return this.save();
 }
 
+GroupSchema.methods.getMembers=function(){
+    return this.members;
+}
+
+GroupSchema.methods.serialize=function(){
+    return this.toJSON();
+}
+
 const Group = mongoose.model('Group',GroupSchema);
 module.exports = Group;
\ No newline at end of file

@@ -18,11 +18,11 @@ const ParticipationSchema = new Schema({
 });
 
 ParticipationSchema.statics.findBySessionId=function(session){
-    return this.find({sessionId:session._id.$oid});
+    return this.find({sessionId:session._id});
 }
 
 ParticipationSchema.statics.findByGroupId=function(group){
-    return this.find({groupId:group._id.$oid});
+    return this.find({groupId:group._id});
 }
 
 ParticipationSchema.methods.addProblem=function(problem){

@@ -9,7 +9,7 @@ const ProblemSchema=new Schema({
     sumbitNum: {type: Number, required: true},
     correctNum: {type: Number, required: true},
     count: { type: Number },
-    category: {type:[String]}
+    category: [{ type:String }],
 },{
     collection: 'problem'
 });

@@ -12,7 +12,7 @@ const SessionSchema = new Schema({
 });
 
 SessionSchema.statics.findByChallengeId=function(challenge){
-    return this.find({challengeId:challenge._id.$oid});
+    return this.find({challengeId:challenge._id});
 }
 
 SessionSchema.methods.getSessionStartDate=function(){

@@ -22,7 +22,7 @@ UserSchema.statics.findByUsername = function (username) {
 };
 
 UserSchema.methods.addFriend=function(friend){
-  this.friendList.push({$oid:friend._id.$oid});
+  this.friendList.push(friend._id);
   return this.save();
 }