Module Sequel::Plugins::Composition
In: 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:

  Date.new(year, 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
  self.day = date.day

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{Date.new(year, month, day) if year || month || day},
    :decomposer=>(proc do
      if d = compositions[:date]
        self.year = d.year
        self.month = d.month
        self.day = d.day
      else
        self.year = nil
        self.month = nil
        self.day = nil
      end
    end)

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.

Methods

apply  

Classes and Modules

Module Sequel::Plugins::Composition::ClassMethods
Module Sequel::Plugins::Composition::InstanceMethods

Public Class methods

Define the necessary class instance variables.

[Source]

    # File lib/sequel/plugins/composition.rb, line 58
58:       def self.apply(model)
59:         model.instance_eval{@compositions = {}}
60:       end

[Validate]