diff --git a/Controllers/SubmissionController.cs b/Controllers/SubmissionController.cs
index b62dfd1..5402683 100644
--- a/Controllers/SubmissionController.cs
+++ b/Controllers/SubmissionController.cs
@@ -70,7 +70,7 @@ namespace _2021_backend.Controllers
 
         public IActionResult GetSessions()
         {
-            var query = _context.Sessions.OrderBy(r => r.Day).ThenBy(r => r.BeginTime).ThenBy(r => r.Place).Where(e => e.Capacity > e.Students.Count + 1).Select(r => new SessionDto
+            var query = _context.Sessions.OrderBy(r => r.Day).ThenBy(r => r.BeginTime).ThenBy(r => r.Place).Where(e => e.Capacity > e.Students.Count).Select(r => new SessionDto
             {
                 BeginTime = r.BeginTime,
                 Day = r.Day,
@@ -98,10 +98,10 @@ namespace _2021_backend.Controllers
             {
                 return StatusCode(400, ApiResponse.Error("INVALID_STUDENT_GUID"));
             }
-            List<SessionDto> timelist;
+            SessionDto time;
             try
             {
-                timelist = JsonSerializer.Deserialize<List<SessionDto>>(selection);
+                time = JsonSerializer.Deserialize<SessionDto>(selection);
             }
             catch (Exception ex)
             {
@@ -109,15 +109,29 @@ namespace _2021_backend.Controllers
                 return StatusCode(400, ApiResponse.Error("MALFORMED_DATA"));
             };
             //get the selected session guids;
-            List<Guid> targetSessions;
+            Guid targetSession = Guid.Empty;
             var sessions = Context.Sessions.ToList();
-            targetSessions = sessions.FindAll((Session s) =>
+            var s = sessions.Find((Session s) =>
             {
-                return timelist.Any(it => it.Day == s.Day && it.BeginTime == s.BeginTime);
-            }).Select(it => it.Guid).ToList();
-            stu.Timelist = targetSessions;
+                return time.Day == s.Day && time.BeginTime == s.BeginTime;
+            });
+            var overwritten = false;
+            if (s.Students.Count >= s.Capacity) return Ok(ApiResponse.Error("OUT_OF_CAPACITY"));
+            if (Context.Sessions.Find(stu.InterviewTime) != null)
+            {
+                var olds = Context.Sessions.Find(stu.InterviewTime);
+                olds.Students.Remove(stu.Guid);
+                Context.SaveChanges();
+                overwritten = true;
+            }
+            targetSession = s.Guid;
+            s.Students.Add(stu.Guid);
+            stu.Timelist = new List<Guid> { targetSession};
             stu.Status = status.已选时间;
-            await Context.SaveChangesAsync();
+            stu.InterviewTime = s.Guid;
+            await TencentSMS.Send(Context, SMSType.TimeSet, stu, "smsbot");
+            Context.SaveChanges();
+            if (overwritten) return Ok(ApiResponse.Success("overwritten"));
             return Ok(ApiResponse.Success("success"));
             //return Ok(ApiResponse.Success("报名结束了"));