Experiment to allow users to use Enumerable-style select on mappers to retrieve the objects they want from the DB

gistfile1.rb

# Attribute class would go into the DB layer
class Attribute
  def initialize name
    @name = name
  end

  # For attribute == value
  # For demo purposes, I didn't feel like implementing other comparators
  def == value
    { @name => value }
  end
end

# Returned by the #select method on mappers
class Selection
  def initialize mapper
    @mapper = mapper
  end

  def method_missing missing_method
    raise "That's not an attribute: #{missing_method}" unless @mapper.attributes.include?(missing_method)
    Attribute.new missing_method
  end
end

class UserMapper
  attr_reader :attributes
  def initialize
    @attributes = [:name, :foo]
  end

  def select &block
    Selection.new(self).instance_eval &block
  end
end

p UserMapper.new.select{ |user| user.name == 'Jamie' }
# => { :name => 'Jamie' } Yaaaay!

p UserMapper.new.select{ |user| user.name == 'Jamie' && user.foo == 'bar' }
# => { :foo => 'bar' } Booo! No mention at all of the first term.