diff --git a/BackEnd/src/main/java/studycave/studycaverestservice/controller/UploadController.java b/BackEnd/src/main/java/studycave/studycaverestservice/controller/UploadController.java
index fb63839..a3f0856 100644
--- a/BackEnd/src/main/java/studycave/studycaverestservice/controller/UploadController.java
+++ b/BackEnd/src/main/java/studycave/studycaverestservice/controller/UploadController.java
@@ -170,7 +170,7 @@ public class UploadController {
             materialDTO.setPermission(material.getPermission());
             materialDTO.setOwner(username);
             if(material.getGroup() != null )
-                if(material.getGroup().getId() != null )
+                if((long)material.getGroup().getId() != 0 )
                     materialDTO.setGroup(groupRepository.findById((long)material.getGroup().getId()).orElse(null).getName());
             materialDTOs.add(materialDTO);
         }
diff --git a/BackEnd/src/main/java/studycave/studycaverestservice/model/flashcardset/Set.java b/BackEnd/src/main/java/studycave/studycaverestservice/model/flashcardset/Set.java
index 529f2fc..b8872d2 100644
--- a/BackEnd/src/main/java/studycave/studycaverestservice/model/flashcardset/Set.java
+++ b/BackEnd/src/main/java/studycave/studycaverestservice/model/flashcardset/Set.java
@@ -52,7 +52,7 @@ public class Set {
 
     @OneToMany(mappedBy="flashcardSet",cascade = CascadeType.ALL)
     @JsonManagedReference
-    List<Flashcard> flashcards;
+    List<Flashcard> flashcards = new ArrayList<>();
 
     @ManyToOne
     @JoinColumn(name="group_id",referencedColumnName="id")
diff --git a/BackEnd/src/main/java/studycave/studycaverestservice/model/studyGroup/GroupService.java b/BackEnd/src/main/java/studycave/studycaverestservice/model/studyGroup/GroupService.java
index c790c5a..0cb4bb6 100644
--- a/BackEnd/src/main/java/studycave/studycaverestservice/model/studyGroup/GroupService.java
+++ b/BackEnd/src/main/java/studycave/studycaverestservice/model/studyGroup/GroupService.java
@@ -122,7 +122,7 @@ public class GroupService {
         StudyGroup group = new StudyGroup();
         group = this.groupRepository.findById(id).orElse(null);
         GroupInfoDto groupInfo = new GroupInfoDto();
-        groupInfo.setId(group.getId());
+        groupInfo.setId((long)group.getId());
         groupInfo.setName(group.getName());
         groupInfo.setDescription(group.getDescription());
         groupInfo.setGroupKey(group.getGroupKey());
@@ -185,7 +185,7 @@ public class GroupService {
         for (StudyGroupMember g : groups) {
             SimpleStudyGroupMemberDTO s = new SimpleStudyGroupMemberDTO();
             s.setName(g.getGroup().getName());
-            s.setId(g.getGroup().getId());
+            s.setId((long)g.getGroup().getId());
             if (g.getIsGroupLeader() == true)
                 s.setRole("OWNER");
             else
diff --git a/BackEnd/src/main/java/studycave/studycaverestservice/model/studyGroup/StudyGroup.java b/BackEnd/src/main/java/studycave/studycaverestservice/model/studyGroup/StudyGroup.java
index 757d3ae..8e6dd53 100644
--- a/BackEnd/src/main/java/studycave/studycaverestservice/model/studyGroup/StudyGroup.java
+++ b/BackEnd/src/main/java/studycave/studycaverestservice/model/studyGroup/StudyGroup.java
@@ -4,6 +4,7 @@ import lombok.AllArgsConstructor;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 import lombok.Setter;
+import studycave.studycaverestservice.model.flashcardset.Set;
 import studycave.studycaverestservice.model.studyGroupMember.StudyGroupMember;
 import studycave.studycaverestservice.model.test.Test;
 import studycave.studycaverestservice.model.userActivity.UserActivity;
@@ -23,7 +24,7 @@ public class StudyGroup {
     @Id
     @Column(name = "id")
     @GeneratedValue(strategy = GenerationType.IDENTITY)
-    private Long id;
+    private int id;
 
     private String name;
 
@@ -36,7 +37,7 @@ public class StudyGroup {
     List<StudyGroupMember> members = new ArrayList<>();
 
     @OneToMany(mappedBy = "group", cascade = CascadeType.ALL)
-    List<StudyGroupMember> flashcardSets = new ArrayList<>();
+    List<Set> flashcardSets = new ArrayList<>();
 
     @OneToMany(mappedBy = "group", cascade = CascadeType.ALL)
     List<Material> materials = new ArrayList<>();