Add validation to avoid overlapping

Add validation in EventSchedule to avoid that several event schedules which belongs to the same schedule and room overlap.

spec/models/event_schedule_spec.rb

@@ -17,5 +17,32 @@ describe EventSchedule do
     it { is_expected.to validate_presence_of(:event) }
     it { is_expected.to validate_presence_of(:room) }
     it { is_expected.to validate_presence_of(:start_time) }
+
+    describe '#not_overlapping' do
+      let!(:event) { create(:event, event_type: create(:event_type, length: 60)) }
+      let!(:event2) { create(:event, event_type: create(:event_type, length: 30), program: event.program) }
+      let!(:schedule) { create(:schedule, program: event.program) }
+      let!(:room) { create(:room, venue: create(:venue, conference: event.program.conference)) }
+      let!(:event_schedule) { create(:event_schedule, schedule: schedule, event: event, room: room, start_time: event.program.conference.start_date.tomorrow.to_time + 60.minutes) }
+
+      describe "can't be scheduled at the same time than other event in the same room" do
+        it 'case 1' do
+          expect(build(:event_schedule, schedule: schedule, event: event2, room: room, start_time: event_schedule.start_time - 15.minutes)).to_not be_valid
+        end
+
+        it 'case 2' do
+          expect(build(:event_schedule, schedule: schedule, event: event2, room: room, start_time: event_schedule.start_time + 45.minutes)).to_not be_valid
+        end
+
+        it 'case 3' do
+          expect(build(:event_schedule, schedule: schedule, event: event2, room: room, start_time: event_schedule.start_time + 15.minutes)).to_not be_valid
+        end
+
+        it 'case 4' do
+          event2.event_type = create(:event_type, length: 120)
+          expect(build(:event_schedule, schedule: schedule, event: event2, room: room, start_time: event_schedule.start_time - 30.minutes)).to_not be_valid
+        end
+      end
+    end
   end
 end