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
  comparable_attributes = collection.first.attribute_names - ignore

  comparable_attributes.each do |attribute|
    attribute_set = collection.collect(&attribute)

    comparable_attributes.delete(attribute) if attribute_set.uniq.size == 1
  end

  attributes = comparable_attributes | always_include

  # build the output set
  # each_with_object requires activesupport; you could easily use inject instead.
  collection.each_with_object({}) do |instance, results|
    # collect the relevant attributes as 2-dimensional array to maintain order
    results[instance.send(key)] = attributes.collect{|attribute| [attribute, instance.send(attribute)] }
  end
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'