[Rails] group validation

hi im new in rails and starting to develop my application which is a
online reservation im just wondering if there is a way in Ruby on
rails to group a validation

ei. i want to validate the uniqueness of date first in the database
and check what are the timeStart and endTime entries in that
particular date and validate its uniqueness(timeStart and endTime)
base on the date? is it possible in validation or do i have to make a
controller or method for it?

to sum up my application can book a reservation in a same date but not
in a same date and time

sorry for my bad english my Reservation database is:
0 id INTEGER 1 1
1 lname varchar(25 1 0
2 fname varchar(25 1 0
3 contactnum integer 1 0
4 email varchar(10 1 0
5 date date 1 '2012-03-0 0
6 timeStart time 1 '---
:hour 0
7 timeEnd time 1 '---
:hour 0
8 numGuest integer 0 5 0
9 totalCost decimal(2) 0 0
10 confirm boolean 0 'f' 0
11 created_at datetime 0 0
12 updated_at datetime 0 0

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