module Sequel::Plugins::Composition

  1. lib/sequel/plugins/composition.rb

The composition plugin allows you to easily define a virtual attribute where the backing data is composed of other columns.

There are two ways to use the plugin. One way is with the :mapping option. A simple example of this is when you have a database table with separate columns for year, month, and day, but where you want to deal with Date objects in your ruby code. This can be handled with:

Album.plugin :composition
Album.composition :date, :mapping=>[:year, :month, :day]

With the :mapping option, you can provide a :class option that gives the class to use, but if that is not provided, it is inferred from the name of the composition (e.g. :date -> Date). When the date method is called, it will return a Date object by calling:, month, day)

When saving the object, if the date composition has been used (by calling either the getter or setter method), it will populate the related columns of the object before saving:

self.year = date.year
self.month = date.month =

The :mapping option is just a shortcut that works in particular cases. To handle any case, you can define a custom :composer and :decomposer procs. The :composer proc will be instance_evaled the first time the getter is called, and the :decomposer proc will be instance_evaled before saving. The above example could also be implemented as:

Album.composition :date,
  :composer=>proc{, month, day) if year || month || day},
  :decomposer=>(proc do
    if d = compositions[:date]
      self.year = d.year
      self.month = d.month =
      self.year = nil
      self.month = nil = nil

Note that when using the composition object, you should not modify the underlying columns if you are also instantiating the composition, as otherwise the composition object values will override any underlying columns when the object is saved.


Public Class

  1. apply

Public Class methods

apply (model)

Define the necessary class instance variables.

[show source]
# File lib/sequel/plugins/composition.rb, line 59
def self.apply(model)
  model.instance_eval{@compositions = {}}