It would be cool if you could define methods on a model that return a filter and chain them, the way you can in ActiveRecord

output.txt

$ ruby sequel_filter_chaining.rb 
I, [2013-02-21T20:16:43.689560 #45206]  INFO -- : (0.000207s) PRAGMA foreign_keys = 1
I, [2013-02-21T20:16:43.689785 #45206]  INFO -- : (0.000045s) PRAGMA case_sensitive_like = 1
I, [2013-02-21T20:16:43.690238 #45206]  INFO -- : (0.000347s) CREATE TABLE `people` (`id` integer NOT NULL PRIMARY KEY AUTOINCREMENT, `gender` varchar(255), `born_on` date)
I, [2013-02-21T20:16:43.691695 #45206]  INFO -- : (0.000569s) PRAGMA table_info('people')
I, [2013-02-21T20:16:43.697758 #45206]  INFO -- : (0.000217s) SELECT sqlite_version() LIMIT 1
I, [2013-02-21T20:16:43.698478 #45206]  INFO -- : (0.000071s) BEGIN
I, [2013-02-21T20:16:43.703150 #45206]  INFO -- : (0.000148s) INSERT INTO `people` (`gender`, `born_on`) VALUES ('m', '1992-02-27')
I, [2013-02-21T20:16:43.706520 #45206]  INFO -- : (0.002898s) SELECT * FROM `people` WHERE (`id` = 1) LIMIT 1
I, [2013-02-21T20:16:43.706702 #45206]  INFO -- : (0.000058s) COMMIT
I, [2013-02-21T20:16:43.707035 #45206]  INFO -- : (0.000033s) BEGIN
I, [2013-02-21T20:16:43.707483 #45206]  INFO -- : (0.000107s) INSERT INTO `people` (`gender`, `born_on`) VALUES ('m', '1995-02-26')
I, [2013-02-21T20:16:43.708044 #45206]  INFO -- : (0.000218s) SELECT * FROM `people` WHERE (`id` = 2) LIMIT 1
I, [2013-02-21T20:16:43.708200 #45206]  INFO -- : (0.000049s) COMMIT
I, [2013-02-21T20:16:43.708432 #45206]  INFO -- : (0.000029s) BEGIN
I, [2013-02-21T20:16:43.708760 #45206]  INFO -- : (0.000079s) INSERT INTO `people` (`gender`, `born_on`) VALUES ('f', '1992-02-27')
I, [2013-02-21T20:16:43.709234 #45206]  INFO -- : (0.000171s) SELECT * FROM `people` WHERE (`id` = 3) LIMIT 1
I, [2013-02-21T20:16:43.709383 #45206]  INFO -- : (0.000046s) COMMIT
I, [2013-02-21T20:16:43.709658 #45206]  INFO -- : (0.000035s) BEGIN
I, [2013-02-21T20:16:43.710075 #45206]  INFO -- : (0.000099s) INSERT INTO `people` (`gender`, `born_on`) VALUES ('f', '1995-02-26')
I, [2013-02-21T20:16:43.710552 #45206]  INFO -- : (0.000144s) SELECT * FROM `people` WHERE (`id` = 4) LIMIT 1
I, [2013-02-21T20:16:43.710685 #45206]  INFO -- : (0.000044s) COMMIT
sequel_filter_chaining.rb:27:in `<main>': undefined method `underage' for #<Sequel::SQLite::Dataset:0x007fb2f8eb9d28> (NoMethodError)

sequel_filter_chaining.rb

require 'logger'
require 'sequel'

db = Sequel.sqlite(':memory:', :logger => Logger.new(STDOUT))

db.create_table :people do
  primary_key :id
  String :gender
  Date :born_on
end

class Person < Sequel::Model(db)
  def self.male
    where(:gender => 'm')
  end
  
  def self.underage
    where{born_on < (Time.now - (60 * 60 * 24 * 365 * 21))}
  end
end

Person.create(:gender => 'm', :born_on => (Time.now - (60 * 60 * 24 * 365 * 21)))
Person.create(:gender => 'm', :born_on => (Time.now - (60 * 60 * 24 * 365 * 18)))
Person.create(:gender => 'f', :born_on => (Time.now - (60 * 60 * 24 * 365 * 21)))
Person.create(:gender => 'f', :born_on => (Time.now - (60 * 60 * 24 * 365 * 18)))

puts Person.male.underage.count