foo_spec.rb

class Foo
    # has_many :bar
    attr_accessor :bar

    def method_missing(method, *args, &block)
        if method.to_s =~ /(\w+)_on/
            raise Exception, "no condition given to filter `#{$1}'" unless args[0]
            result = self.send($1.to_sym)
            args[0].each { |field, value|
                result = result.select { |item|
                    item.send(field) == value
                }
            }
            result
        end
    end
end

class Bar
    # belongs_to :foo
    attr_accessor :number, :texture

    def initialize(params={})
        @number  = params[:number]
        @texture = params[:texture]
    end
end

describe Foo do
    before(:each) do
        @foo = Foo.new
        @foo.bar = (1..9).map { |i| Bar.new(:number => i, :texture => (i>5 ? "wood": "iron")) }
    end
    it "has a bar number is 8" do
        @foo.should have(1).bar_on(:texture => "wood", :number => 8)
        @foo.bar_on(:texture => "wood", :number => 8).length.should == 1
    end
    it "has no wood bar number is 4" do
        @foo.should have(0).bar_on(:texture => "wood", :number => 4)
    end
end