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

apply(model)

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]