Is that a variable a Symbol, a Method, why does this have a colon and that doesn't?

Question

Inconsistent naming conventions in Rails are confusing me. It seems like the syntax is all over the place. Here are some examples:

Why are there commas in the migration below? And, why doesn't the keyword default have a colon before it? What is this default keyword, a method, or a variable, a symbol? What is that thing?:

add_column :zombies, :rotting, :boolean, default: false

Here is another example:

Why is age not :age (with a colon)? Why is make_rotting called with a ":" before it?

 class Zombie < ActiveRecord::Base
       before_save :make_rotting

   def make_rotting
      if age > 20
         self.rotting = true
      end
    end
 end

Answer 1

Ruby can be hard for Java and PHP people. :)

In Ruby, not everything is what it appears to be. Take this, for example:

before_save :make_rotting

This is a method call, sure. But it's not the make_rotting method that is called. It's the before_save (:make_rotting is its parameter). This is a so-called hook in ActiveRecord. before_save will take a method name as a parameter and will dynamically call it when the moment comes.

if age > 20

Here age is a method call, not a symbol. It could be written as:

if age() > 20

but the parentheses are optional. And lastly:

add_column :zombies, :rotting, :boolean, default: false

This method takes four parameters, the last of which is a hash. The hash uses the new Ruby 1.9 syntax. Previously it would be written like this (with the colon in the right place, and all):

add_column :zombies, :rotting, :boolean, :default => false

You should read a good book on Ruby programming, instead of scraping pieces of knowledge from Stack Overflow posts. :)