# HG changeset patch
# Parent 8d5573b704b2867788dd6895503f1871c2976a29
# Parent  9eb163e5da677a0d09e01a354ba56697b576a4bc

diff --git a/openacademy/models.py b/openacademy/models.py
--- a/openacademy/models.py
+++ b/openacademy/models.py
@@ -39,3 +39,20 @@ class Session(models.Model):
                 r.taken_seats = 0.0
             else:
                 r.taken_seats = 100.0 * len(r.attendee_ids) / r.seats
+
+    @api.onchange('seats', 'attendee_ids')
+    def _verify_valid_seats(self):
+        if self.seats < 0:
+            return {
+                'warning': {
+                    'title': "Incorrect 'seats' value",
+                    'message': "The number of available seats may not be negative",
+                },
+            }
+        if self.seats < len(self.attendee_ids):
+            return {
+                'warning': {
+                    'title': "Too many attendees",
+                    'message': "Increase seats or remove excess attendees",
+                },
+            }