Bundler - cannot load 2 different versions of the exact same gem?

Question

I've watched this presentation on how Bundler works and one of the reasons Bundler was invented was to solve 'activation errors', for eg:

LoadError: can't activate rack (~> 1.0.0, runtime) for ['actonpack-2.3.5'], already activated rack-1.1.0 for ['thin-1.2.7']

Does this basically means that you cannot run 2 different versions of the exact same gem at a same time, in a single Ruby process?

Answer 1

You cannot have two different versions of the same gem loaded the same program, because they could conflict and override each other's methods. For example, suppose you have a gem which contains the following:

# In gem version 1.0
class AneMaria
  def self.name
    "Ane"
  end
end

######

# In gem version 2.0
class AneMaria
  def self.name
    "Maria"
  end
end

And then in your code, if you call AneMaria.name, what should it return??

One of the reasons tools like bundler were created was to prevent this from happening accidentally -- so no, you cannot specify 2 different versions of a gem to load simultaneously. (Also, I don't think you'll ever want to do that!)

See also this post, which explains things a little more.