그룹 탈퇴 API 로직 수정

src/main/java/com/petlog/petgroup/entity/PetGroupMember.java

@@ -45,4 +45,8 @@ public PetGroupMember(
         this.petGroup = petGroup;
         this.isGroupOwner = isGroupOwner;
     }
+
+    public void updateIsGroupOwner(final boolean isGroupOwner) {
+        this.isGroupOwner = isGroupOwner;
+    }
 }

src/main/java/com/petlog/petgroup/repository/PetGroupMemberRepository.java

@@ -30,4 +30,6 @@ public interface PetGroupMemberRepository extends JpaRepository<PetGroupMember,
     boolean existsByMemberAndPetGroup(final Member member, final PetGroup group);
 
     Optional<PetGroupMember> findByMemberAndPetGroup(final Member member, final PetGroup group);
+
+    Optional<List<PetGroupMember>> findAllByPetGroup(final PetGroup group);
 }

src/main/java/com/petlog/petgroup/service/PetGroupService.java

@@ -23,6 +23,7 @@
 import org.springframework.transaction.annotation.Transactional;
 
 import java.util.List;
+import java.util.concurrent.ThreadLocalRandom;
 
 @RequiredArgsConstructor
 @Service
@@ -114,7 +115,11 @@ public GetMyPetGroupDto getMyPetGroups(final Long memberId) {
     public void leavePetGroup(final Long memberId, final Long groupId) {
         final Member member = getMember(memberId);
         final PetGroup petGroup = getPetGroup(groupId);
-        validateMemberInPetGroup(member, petGroup);
+        final PetGroupMember petGroupMember = getPetGroupMember(member, petGroup);
+
+        if(petGroupMember.isGroupOwner()) {
+            updatePetGroupOwner(petGroupMember, petGroup);
+        }
 
         petGroupMemberRepository.deleteByMemberIdAndGroupId(memberId, groupId);
     }
@@ -124,10 +129,31 @@ private PetGroup getPetGroup(final Long petGroupId) {
             .orElseThrow(() -> new IllegalArgumentException("존재하지 않는 그룹입니다."));
     }
 
-    private void validateMemberInPetGroup(final Member member, final PetGroup petGroup) {
-        if(!petGroupMemberRepository.existsByMemberAndPetGroup(member, petGroup)) {
-            throw new IllegalArgumentException("그룹에 존재하지 않는 회원입니다.");
+    private PetGroupMember getPetGroupMember(final Member member, final PetGroup petGroup) {
+        return petGroupMemberRepository.findByMemberAndPetGroup(member, petGroup)
+            .orElseThrow(() -> new IllegalArgumentException("그룹에 존재하지 않는 회원입니다."));
+    }
+
+    private void updatePetGroupOwner(final PetGroupMember currentOwner, final PetGroup petGroup) {
+        List<PetGroupMember> petGroupMembers = getPetGroupMembers(petGroup);
+
+        final List<PetGroupMember> candidates = petGroupMembers.stream()
+            .filter(member -> !member.equals(currentOwner))
+            .toList();
+
+        if (candidates.isEmpty()) {
+            return;
         }
+
+        final int index = ThreadLocalRandom.current().nextInt(candidates.size());
+        final PetGroupMember newOwner = candidates.get(index);
+
+        newOwner.updateIsGroupOwner(true);
+    }
+
+    private List<PetGroupMember> getPetGroupMembers(final PetGroup petGroup) {
+        return petGroupMemberRepository.findAllByPetGroup(petGroup)
+            .orElseThrow(() -> new IllegalArgumentException("해당 그룹에는 회원이 존재하지 않습니다."));
     }
 
     @Transactional(readOnly = true)
@@ -139,6 +165,12 @@ public GetJoinCodeDto getJoinCode(final Long memberId, final Long groupId) {
         return new GetJoinCodeDto(petGroup.getJoinCode());
     }
 
+    private void validateMemberInPetGroup(final Member member, final PetGroup petGroup) {
+        if(!petGroupMemberRepository.existsByMemberAndPetGroup(member, petGroup)) {
+            throw new IllegalArgumentException("그룹에 존재하지 않는 회원입니다.");
+        }
+    }
+
     @Transactional(readOnly = true)
     public GetNoteDto getNote(final Long memberId, final Long groupId) {
         final Member member = getMember(memberId);
@@ -159,11 +191,6 @@ public void updateNote(final Long memberId, final Long groupId, final String not
         petGroup.updateNote(note);
     }
 
-    private PetGroupMember getPetGroupMember(final Member member, final PetGroup petGroup) {
-        return petGroupMemberRepository.findByMemberAndPetGroup(member, petGroup)
-            .orElseThrow(() -> new IllegalArgumentException("그룹에 존재하지 않는 회원입니다."));
-    }
-
     private void validateMemberIsGroupOwner(final PetGroupMember petGroupMember) {
         if(!petGroupMember.isGroupOwner()) {
             throw new IllegalArgumentException("참고사항 수정은 그룹장만 할 수 있습니다.");