Ruby (and Rails) nested module syntax

Question

I'm wondering what the difference is between the following two modules

# First Example module Parent   module Child   end end 

and

# Second Example module Parent::Child end 

Using the 2nd method, it appears as though the Parent module must be previously defined, otherwise I get an 'uninitialized constant' error

Given this, what is the preferred way of defining modules such as this and then adding nested children with regards to syntax and file structure (ie. folders etc). Reference to a Rails way would be greatly appreciated.

Are these two examples for all intents and purposes equivalent?

Answer 1

In the first example, it defines the Parent module and then the Child module. The second example, as you say yourself, must have the Parent module defined before hand. At the expense of one more line of code, you ensure that the module that you're nesting under by using your first example is always going to be defined.

For a Rails example let's look into the railties/lib/rails/engine.rb file which re-opens the Rails module and then defines an Engine class inside of it. This could have been done with simply:

class Rails::Engine 

But instead perhaps for the reasons stated above and perhaps also for clarity's sake, the module was defined first, then the class inside.