发送消息，一次性最多只能在一个课程下发消息

src/main/java/com/teaching/backend/common/ErrorCode.java

@@ -33,6 +33,7 @@ public enum ErrorCode {
     CONTENT_EXISTS(41000, "内容存在"),
     STUDENT_ROLE_CANT_MULTIPLESEND(41001, "学生用户不能群发，只能单发"),
     LEAST_ONE_USER(41002, "最少选择一个用户"),
+    MOST_ONE_COURSE(41003, "您一次性最多只能在一个课程下发消息！"),
     KNOWS_EXISTS(41001, "存在关联的知识点"),
     ADD_RELATION_FAILED(41002, "添加关系失败"),
 

src/main/java/com/teaching/backend/service/impl/message/MessagesServiceImpl.java

@@ -59,6 +59,13 @@ public class MessagesServiceImpl extends ServiceImpl<MessagesMapper, SysMessages
             throw new BusinessException(ErrorCode.LEAST_ONE_USER);
         }
 
+        List<String> courseIds = Arrays.stream(sysMessageDTO.getCourseId().split(","))
+                .map(String::valueOf)
+                .collect(Collectors.toList());
+        if (courseIds.size()>1){
+            throw new BusinessException(ErrorCode.MOST_ONE_COURSE);
+        }
+
         // 4.判断是什么用户，只有教师可以群发，学生只能单发
         String roleId = umsUserMapper.selectOne(new LambdaQueryWrapper<>(UmsUser.class)
                 .eq(UmsUser::getId, sysMessageDTO.getSenderId())).getRoleId();