bundler, rubygems and dynamic use of gems

I'm developing a gem with uses bundler for its dependency management.

Now I want to use some gems but only when they are already installed in the system (shown by gem list), so I cannot add them to the Gemfile. Doing a simple

require("rmagick") 

does not work because bundler seems to hide all gems not listed in the Gemfile. A simple test.rb without bundler and just containing

require "rubygems"
require "rmagick

works without any problem开发者_Go百科s.

Anybody knows how to require a gem which is not listed in the Gemfile?

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As far as I know you have to include it in your Gemfile, or include it prior to using bundler. If you include it after you get an error. There may be a way to turn that off but I can't find it.