Method namespace clashing when running rake tasks in Rails

Question

Using Rails 2.3.10 If my lib/tasks looks like this

lib/tasks
- a.rake
- b.rake

a.rake looks like this:

namespace :a do
    desc "Task A"
    task(:a=>:environment)do
      msg('I AM TASK A')
    end

    def msg(msg)
      puts "MSG SENT FROM Task A: #{msg}"
    end
end

b.rake looks like this

namespace :b do
    desc "Task B"
    task(:b=>:environment)do
      msg('I AM TASK B')
    end

    def msg(msg)
      puts "MSG SENT FROM Task B: #{msg}"
    end
end

Then when I run task a

rake a RAILS_ENV=sandbox

The output is "MSG SENT FROM Task B: I AM TASK A"

So the msg() helper method defined in a.rake does not get called. Rather that defined in b.rake gets called. (What's more, if I have a c.rake - then it's msg helper method gets called when I run task a.

Is this method namespace clashing known behavior? I would have thought the namespacing would have prevented this.

Thanks

Answer 1

What you observe is that methods in rake files namespaces redefine previously defined methods with the same name. The reason for this is that Rake namespaces are very different than Ruby namespaces (classes or modules), in fact they only serve as a namespace for the names of the tasks defined in them, but nothing else. Thus, task a becomes task a:a if placed in the a namespace but other code outside the tasks shares the same global namespace.

This fact, together with the fact that Rake loads all tasks before running the given task, explains why the method gets redefined.

TL;DR: Solutions / hints for the name clashes

You cannot expect that two methods with the same name (or any other code) placed inside separate namespaces but outside tasks will work properly. Nevertheless, here are a couple of hints to solve such situation:

• Place the methods inside the tasks. If both msg methods were defined inside the a:a and b:b tasks, then both rake tasks would run properly and would display the expected messages.

• If you need to use the code from the rake's namespace in multiple rake tasks, extract the methods / code to a real Ruby namespace, such as two modules, and include the code in the tasks that need it. Consider this rewrite of the sample rakes:

  # lib/tasks/a.rake:
  module HelperMethodsA
    def msg(msg)
      puts "MSG SENT FROM Task A: #{msg}"
    end
  end
  
  namespace :a do
    desc "Task A"
    task(:a => :environment) do
      include HelperMethodsA
      msg('I AM TASK A')
    end
  end
  
  # lib/tasks/b.rake:
  module HelperMethodsB
    def msg(msg)
      puts "MSG SENT FROM Task B: #{msg}"
    end
  end
  
  namespace :b do
    desc "Task B"
    task(:b => :environment) do
      include HelperMethodsB
      msg('I AM TASK B')
    end
  end
  

  Because the two modules have different names and because they are included in the respective tasks, both rake tasks will again run as expected.

Now, let's prove the above claims with the help of the source code...

Proof that Rake loads all tasks first and why it does so

This one's easy. In the main Rakefile you'll always find the following line:

Rails.application.load_tasks

This method eventually calls the following code from the Rails engine:

def run_tasks_blocks(*) #:nodoc:
  super
  paths["lib/tasks"].existent.sort.each { |ext| load(ext) }
end

Thus, it searches the lib/tasks directories for all rake files and loads them one after each other, in a sorted order. That's why the b.rake file will be loaded after a.rake and whatever is inside it will potentially redefine the code from a.rake and all previously loaded rake files.

Rake has to load all rake files simply because the rake namespace names do not have to be the same as rake filenames, so the rake filename cannot be inferred from the task / namespace name.

Proof that rake's namespaces don't constitute a real Ruby-like namespace

Upon loading the rake files, the Rake DSL statements get executed, and also the namespace method. The method takes the block of code defined inside it and executes it (using yield) in the context of the Rake.application object which is a singleton object of the Rake::Application class that is shared among all rake tasks. There is no dynamic module / class created for the namespace, it's just executed in the main object context.

# this reuses the shared Rake.application object and executes the namespace code inside it
def namespace(name=nil, &block)
  # ...
  Rake.application.in_namespace(name, &block)
end

# the namespace block is yielded in the context of Rake.application shared object
def in_namespace(name)
  # ...
  @scope = Scope.new(name, @scope)
  ns = NameSpace.new(self, @scope)
  yield(ns)
  # ...
end

See the relevant sources here and here.

Rake tasks DO constitute ruby namespaces

With Rake tasks themselves, the situation is different, though. For each task, a separate object of the Rake::Task class (or of a similar class) is created and the task code is run inside this object's context. The creation of the object is done in the intern method in the task manager:

def intern(task_class, task_name)
  @tasks[task_name.to_s] ||= task_class.new(task_name, self)
end

A quote form the Rake's author

Finally, all this is confirmed by this interesting discussion on github that deals with a very similar and related issue, and from which we can quote Jim Weirich, the original author of Rake:

Since namespaces don't introduce real method scopes, the only real possibility for a scope is the DSL module.

...

Perhaps, someday, the Rake namespaces will become full class scoped entities with a place to hang lazy let definitions, but we are not there yet.