Defining methods in Ruby

September 6, 2009 at 6:50 pm Leave a comment

1 – Hashes in method calls

You must have seen this syntax in Rails a lot, for example when using this Paperclip plugin, you would put this in your model:

has_attached_file :photo, :url => "/:class/:attachment/:id/:style/:basename.:extension"

While it looks pretty natural, you might have harder time understanding that has_attached_file is really a method, and there’s a considerable amount of trickery involved to make this method call much easier on your eyes. This post will not only help you read the Ruby code better, but actually create the methods with flexible parameters yourself.

Consider this equivalent call:

has_attached_file(:photo, {:url => "/:class/:attachment/:id/:style/:basename.:extension"})

Now, it’s more clear that has_attached_file method passes 2 values, where a second value is a hash. Allowing hashes in method calls gives a great freedom for both the developer and user of the method – you do not need to remember what’s the correct sequence of the parameters for the method and it’s possible to omit some parameters altogether. So, instead of some_method(first, second, third), you can use a more flexible some_method :second => second, :first => first, :third => third.

Do note, that if you omit the round brackets, you should also omit the curly brackets, so that the hash is not confused for a block, which also uses the curly brackets.

Actually, if you look at how the method was defined in paperclip plugin, you will see this:

def has_attached_file name, options = {}
  ...
end

This method definition (def) uses the fact that Ruby makes brackets optional even for method definitions, so it’s equivalent to this definition:

def has_attached_file(name, options = {})
  ...
end

And what it means is that has_attached_file method expects 2 parameters. The one called options is a hash and it defaults to an empty hash. Actually defaults are quite powerful in Ruby – you can pass any arbitrary Ruby expression to them and call any variables in scope and link to any other parameters. You could have said, for example – def has_attached_file name, options = name.size.

So, how would you access those values passed in the hash? This is very easy, you would do something like this:

def has_attached_file(name, options = {})
  size = options[:size] || 0
  file = options[:file] || "undefined"
end

So, you would check for a hash element with a key :file and if it exists, assign it to the file local variable. If it does not exist, you will (by using ||) set a default.

2 – How many parameters do you want? – Splat arguments

Another common way to define a method is like this:

def some_method(*args)
  ...
end

This means that however many arguments can be provided and they will all be moved into array called args, so that you can access them as a normal array. You can also do something like:

def some_method(first, *args)
  ...
end

This will move the first parameter to first local var, all others (if any) will go into the args array.

Splatting comes in handy, if you want to define a method for your class that does a slight modification of the superclass’ method and then passes all the arguments to the superclass:

Class Child < Parent
  def some_method(*args)
     # your code here
     super
  end
end

If you use super without parameters, it will pass whatever parameters your method received to the parent class. And you can actually further shorten the way you define the method, by just using an asterisk in def, like this:

  def some_method(*)
     # your code here
     super
  end

3. Define your own method on fly?

Another thing you will notice a lot in Rails, is the possibility to define the methods on the fly. And great thing about it is that you can create the method name using any variables and Ruby expressions. Here’s another example from the Paperclip:

define_method "#{name}=" do
  ...
end

This will create a method using the name variable, so that you can have access to this method based on the name of your choice. For more info you can see Ruby docs.

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