Ruby define_method

Question

I have the following test which I must pass:

def test_can_find_by_arbitrary_fields
  assert @library.respond_to? :find_by_artist
  assert [email protected]_to?(:find_by_bitrate)
  @library.add_song({ :artist => 'Green Day',
                     :name => 'American Idiot',
                     :bitrate => 192 })
  assert @library.respond_to?(:find_by_bitrate)
end

and I am not sure how I can do it.

I tried doing:

def respond_to?(method)
     if self.public_methods.include? method
         true
     elsif (method == :find_by_bitrate)
         define_method :find_by_bitrate, ->(default = nrb)  { @songs.select |a| a[:bitrate] == nrb }       
         false
     else
         false
end

but it says "define_method is undefined". Are there any ways I can define the find_by_bitrate method?

Are you sure you want to be redefining respond_to?? I suspect that's very much not what you want to do. At the very least you almost definitely do not want to be defining a method in respond_to?. — Andrew Marshall
– Andrew Marshall, Commented Oct 31, 2012 at 22:02
Not really. But I can't think of any other way of inserting the find_by_bitrate after the first time it's not found. — Casteurr
– Casteurr, Commented Oct 31, 2012 at 22:03
But respond_to? isn't called when trying to call a method on an object, so unless your actual app code calls respond_to? it won't define find_by_bitrate. Are you using Ruby 1.9? — Andrew Marshall
– Andrew Marshall, Commented Oct 31, 2012 at 22:05

Dave Newton · Accepted Answer · 2012-10-31 23:36:49Z

3

You may define methods the first time they're called in method_missing.

Whether or not you should is open to some debate, but it's a better option than respond_to?.

class Foo
  def method_missing(sym)
    puts "Method missing; defining."
    self.class.send(:define_method, sym) do
      puts "Called #{sym}."
    end
  end
end

Sanity check:

f = Foo.new
=> #<Foo:0x007fa6aa09d3c0>
f.wat
=> Method wat missing; defining.
f.wat
=> Called wat.
f2 = Foo.new
=> Called wat.

answered Oct 31, 2012 at 23:36

Dave Newton

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Comments

Community · Accepted Answer · 2017-05-23 12:33:47Z

1

I don't think you should be redefining respond_to? method. The point of the test is (probably) that the @library object should have a find_by_artist method defined and no find_by_bitrate until you add a song with a bitrate. I.e. the add_song method should define method find_by_bitrate when it sees a song with a bitrate (?).

Also, define_method is a private method of Class. Above, you're trying to call it from an instance method. See "Ruby: define_method vs. def", there's more on this stuff.

edited May 23, 2017 at 12:33

CommunityBot

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answered Oct 31, 2012 at 22:06

szeryf

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Comments

dbenhur · Accepted Answer · 2012-10-31 23:25:00Z

There's a lot of info missing to properly answer this. The test implies that find_by_artist is always defined even when @library is empty, but that there are dynamic methods available on other attributes (eg: bitrate) that are valid only when library contains a record with such a method.

One should not redefine respond_to? in any case. There is an explicit hook method for answering respond_to? for dynamic methods: Object#respond_to_missing?.

So a simple way to make your test pass is to be sure the @library object has a concrete method #find_by_artist and a respond to hook that checks whether any of it's elements a have the requested attribute. If I assume @library is a collection object Library which keeps an enumeration of songs in @songs

class Library
  def find_by_artist artist
    @songs.select { |song| song['artist'] == artist }
  end

  def method_missing meth, arg
    m = /^find_by_(.+)$/.match meth.to_s
    return super unless attr = m && m[1]

    @songs.select { |song| song[attr] ==  arg }
  end

  def respond_to_missing? meth, include_private
    m = /^find_by_(.+)$/.match meth.to_s
    return super unless attr = m && m[1]

    @songs.any? { |song| song.has_key? attr }
  end
end

This has a performance problem in that respond_to? now incurs a search of all the songs. One could optimize by keeping a set of the union of all attributes contained in @songs and updating it in methods which add/update/delete elements in the collection.

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