ActiveRecord-style array filtering

array_ext.rb

#!/usr/bin/env ruby

# ArrayExt shows how to extend Array to support activerecord-style
# filter chaining.

class ArrayExt
  
  # When we create the ArrayExt, we store the actual array
  # we want to operate on internally
  def initialize(finder)
    # Store delegation object
    @array = finder.to_a
  end
    
  # We don't use Ruby's built-in delegation, because then filters
  # would return Array objects, not ArrayExt objects, so we have to
  # delegate manually through method_missing
  def method_missing(*args, &block)
    # Delegate to wrapped array
    r = @array.send(*args, &block)
    # Return an ArrayExt, not a standard array, if the result is an array
    if r.is_a?(Array)
      ArrayExt.new(r)
    else
      r
    end
  end
    
  # We can then create our filters. Calling 'self' is important here.
  # ArrayExt#select doesn't exist, and Kernel#select will be called
  # instead, before ArrayExt#method_missing, so we have to force it.
  def single_digits
    self.select {|x| x < 10}
  end

  def more_than_five
    self.select {|x| x > 5}
  end

end

x = ArrayExt.new [3,7,12,1,34]

puts x.single_digits.inspect
# => #<ArrayExt:0x007fef688461d8 @array=[3, 7, 1]>

puts x.more_than_five.inspect
# => #<ArrayExt:0x007fef68845e18 @array=[7, 12, 34]>

# The filters can be chained just like you would with activerecord - win!

puts x.single_digits.more_than_five.inspect
# => #<ArrayExt:0x007fef68845e18 @array=[7]>