feat/chatroom-dto-add-lastChatCreated: 채팅방 GET 요청 시 마지막 채팅 시간 DTO에 추가

src/main/java/com/dife/api/model/dto/ChatroomResponseDto.java

@@ -16,4 +16,6 @@ public interface ChatroomResponseDto {
 	Set<MemberRestrictedResponseDto> getMembers();
 
 	String getLastChat();
+
+	LocalDateTime getLastChatCreated();
 }

src/main/java/com/dife/api/model/dto/GroupChatroomResponseDto.java

@@ -52,4 +52,6 @@ public class GroupChatroomResponseDto implements ChatroomResponseDto {
 	private Boolean isEntered;
 
 	private String lastChat;
+
+	private LocalDateTime lastChatCreated;
 }

src/main/java/com/dife/api/model/dto/SingleChatroomResponseDto.java

@@ -28,4 +28,6 @@ public class SingleChatroomResponseDto implements ChatroomResponseDto {
 	private Long unreadChatsCount = 0L;
 
 	private String lastChat;
+
+	private LocalDateTime lastChatCreated;
 }

src/main/java/com/dife/api/service/ChatroomService.java

@@ -411,6 +411,7 @@ public SingleChatroomResponseDto getSingleChatroom(Long id, String memberEmail)
 
 		changeChatroomMemberToMemberSet(chatroom, responseDto);
 		responseDto.setLastChat(getLastChat(chatroom, member));
+		responseDto.setLastChatCreated(getLastChatCreated(chatroom, member));
 		responseDto.setUnreadChatsCount(getChatCounts(chatroomMember));
 		return responseDto;
 	}
@@ -475,6 +476,22 @@ private String getLastChat(Chatroom chatroom, Member member) {
 		return chats.isEmpty() ? "" : chats.get(0).getMessage();
 	}
 
+	private LocalDateTime getLastChatCreated(Chatroom chatroom, Member member) {
+		ChatroomMember chatroomMember =
+				chatroomMemberRepository
+						.findByChatroomAndMember(chatroom, member)
+						.orElseThrow(ChatroomNotFoundException::new);
+
+		LocalDateTime last_joined_at = chatroomMember.getLastJoinedAt();
+
+		List<Chat> lastChats = chatroom.getChats().stream()
+				.filter(chat -> chat.getCreated().isAfter(last_joined_at))
+				.sorted(Comparator.comparing(Chat::getCreated).reversed())
+				.toList();
+
+		return lastChats.isEmpty() ? null : lastChats.get(0).getCreated();
+	}
+
 	public GroupChatroomResponseDto getGroupChatroom(Long id, String memberEmail) {
 
 		Chatroom chatroom = chatroomRepository.findById(id).orElseThrow(ChatroomNotFoundException::new);