Filter a set of attributes on a collection to only show the varying attributes

set-matching.rb

#! /usr/bin/ruby

# data set:

# id | name | score | postal | color
#  1 | john |    14 | 12345  | blue
#  2 | jane |    10 | 12345  | blue
#  3 | jeff |     6 | 12345  | green
 
# define a simple class for holding the dataset
class Thing
  attr_reader :id, :name, :score, :postal, :color
  
  #quick & dirty setup
  def initialize(id, name, score, postal, color)
    @id     = id
    @name   = name
    @score  = score
    @postal = postal
    @color  = color
  end
  
  #something that looks more like ActiveRecord
  def attribute_names
    [:id, :name, :score, :postal, :color]
  end
  
  def attributes
    { :id     => @id,
      :name   => @name,
      :score  => @score,
      :postal => @postal,
      :color  => @color
    }
  end
  
end

# the meat
def diff_collection(collection, key, always_include = [], ignore = [])
  
  # find some more attributes to ignore
  (collection.first.attribute_names - always_include - ignore).each do |attribute|
    attribute_set = collection.collect(&attribute)
    ignore << attribute if attribute_set.all?{|n| n == attribute_set[0]}
  end
  
  #set the attributes in order, always_include's first, then the rest of the comparables
  comparable_attributes = (always_include + (collection.first.attribute_names - always_include - ignore))
  
  #build the output set
  results = {}
  collection.each do |instance|
    # collect the relevant attributes as 2-dimensional array to maintain order
    attributes = comparable_attributes.collect{|attribute| [attribute, instance.send(attribute)] }
    #add to the result set using the key attribute 
    results[instance.send(key)] = attributes
  end
  results
end

#construct the collection
things = [ Thing.new(1, 'john', 14, '12345', 'blue'),
           Thing.new(2, 'jane', 10, '12345', 'blue'),
           Thing.new(3, 'jeff',  6, '12345', 'green') ]

# do it already!
results = diff_collection(things, :id, [:name], [:id])

#print out something nice
results.each do |key, attributes|
  puts "[#{key}] #{attributes.collect{|a| "#{a[0]} => '#{a[1]}' "} }"
end

# output:

# [1] name => 'john' score => '14' color => 'blue' 
# [2] name => 'jane' score => '10' color => 'blue' 
# [3] name => 'jeff' score => '6' color => 'green'