This project rocks and uses MIT-LICENSE.
If you are developing Rails 3 app, let say some meeting planner and you can't save records which have time overlap.
Add to your gemfile
gem 'validates_overlap'
In your model
validates :starts_at, :ends_at, :overlap => true
validates :starts_at, :ends_at, :overlap => {:scope => "user_id"}
validates :starts_at, :ends_at, :overlap => {:exclude_edges => "starts_at"}
validates :starts_at, :ends_at, :overlap => {:exclude_edges => ["starts_at", "ends_at"]}
validates :starts_at, :ends_at, :overlap => {:message_title => "Some validation title", :message_content => "Some validation message"}
Example describes valildatation of user, positions and time slots. User can't be assigned 2 times on position which is under time slot with time overlap.
class Position < ActiveRecord::Base
belongs_to :time_slot
belongs_to :user
validates "time_slots.starts_at", "time_slots.ends_at",
:overlap => {
:query_options => {:includes => :time_slot},
:scope => { "positions.user_id" => proc{|position| position.user_id} }
}
end
class ActiveMeeting < ActiveRecord::Base
validates :starts_at, :ends_at, :overlap => {:query_options => {:active => true}}
scope :active, where(:is_active => true)
end