Kata de api fluida

gistfile1.rb

class User
  attr_reader :name, :age
  def initialize(name, age)
    @name, @age = name, age
  end
end

class Filter
  attr_reader :method, :value
  def initialize(method, value)
    @method, @value = method, value
  end
end

class Select
  instance_methods.each do |method|
    undef_method method unless method.to_s =~ /(^__|^send|^object_id)/
  end

  def self.[](query)
    Select.new(query)
  end

  def initialize(query)
    @query = query
  end

  def from(list)
    @from = list
    self
  end

  def where(where)
    @where = where
    self
  end

  def is(filter)
    @filter = Filter.new('==', filter)
    self
  end

  def is_greater_than(filter)
    @filter = Filter.new('>', filter)
    self
  end

  def is_lower_than(filter)
    @filter = Filter.new('<', filter)
    self
  end

  def method_missing(method_name, *args)
    result = execute
    result.send(method_name, *args)
  end

  private
  def execute
    return nil if @from.nil? || @from.empty?

    @from = @from.select do |e|
      e.send(@where).send(@filter.method, @filter.value)
    end if @where && @filter

    @from.map {|e| e.send(@query) }
  end
end

describe Select do
  it "return nil when the list is empty" do
    Select['name'].should be_nil
  end

  it "return the name from an element from the list" do
    Select["name"].from([User.new("foo", 23)]).should == ["foo"]
  end

  it "return the age from an element from the list" do
    Select['age'].from([User.new('foo', 23)]).should == [23]
  end

  it "returns the different names for all the elements in the list" do
    users = [User.new('foo', 23),
             User.new('bar', 34)]
    Select["name"].from(users).should == ['foo', 'bar']
  end

  it "filters the results by the where expression" do
    users = [User.new('foo', 23),
             User.new('bar', 34)]
    Select['name'].from(users).where("age").is(23).should == ['foo']
  end

  it "filters the results greater than an age" do
    users = [User.new('foo', 23),
             User.new('bar', 34)]
    Select['name'].from(users).where('age').is_greater_than(23).should == ['bar']
  end

  it "filters the results lower than an age" do
    users = [User.new('foo', 23),
             User.new('bar', 34)]
    Select['name'].from(users).where('age').is_lower_than(24).should == ['foo']
  end
end