I'm asking this with the intention of also supplying the answer, since I had a lot of trouble figuring out what was going wrong, and Google wasn't much help - however, since this is StackOverflow, I'm sure that someone will answer it more competently or improve the answer.
I needed to upgrade a gem for a rails application that I'm working on, which introduced two new dependencies, one of which is mime-types. Working locally, I added the following line to config/environment.rb:
config.gem 'mime-types', :version => '1.17'
For deployment on our server, it's necessary for all gem dependencies to be installed in vendor/gems, so I then installed the mime-types gem into the current rvm gemset with:
gem install mime-types --version 1.17
... and unpacked a version of that into vendor/gems with:
rake gems:unpack
rake gems:unpack:dependencies
However, after deploying on the server, I found that when rake db:migrate was run, the following error appeared, indicating that the mime-types gem couldn't be found (stack trace partially elided):
no such file to load -- mime-types
/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `require'
[...]
/lib/tasks/misc.rake:4
/usr/lib/ruby/1.8/rake.rb:636:in `call'
/usr/lib/ruby/1.8/rake.rb:636:in `execute'
/usr/lib/ruby/1.8/rake.rb:631:in `each'
/usr/lib/ruby/1.8/rake.rb:631:in `execute'
/usr/lib/ruby/1.8/rake.rb:597:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/lib/ruby/1.8/rake.rb:590:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/rake.rb:607:in `invoke_prerequisites'
/usr/lib/ruby/1.8/rake.rb:604:in `each'
/usr/lib/ruby/1.8/rake.rb:604:in `invoke_prerequisites'
/usr/lib/ruby/1.8/rake.rb:596:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/lib/ruby/1.8/rake.rb:590:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/rake.rb:583:in `invoke'
/usr/lib/ruby/1.8/rake.rb:2051:in `invoke_task'
/usr/lib/ruby/1.8/rake.rb:2029:in `top_level'
/usr/lib/ruby/1.8/rake.rb:2029:in `each'
/usr/lib/ruby/1.8/rake.rb:2029:in `top_level'
/usr/lib/ruby/1.8/rake.rb:2068:in `standard_exception_handling'
/usr/lib/ruby/1.8/rake.rb:2023:in `top_level'
/usr/lib/ruby/1.8/rake.rb:2001:in `run'
/usr/lib/ruby/1.8/rake.rb:2068:in `standard_exception_handling'
/usr/lib/ruby/1.8/rake.rb:1998:in `run'
/usr/bin/rake:28
no such file to load -- mime-types
/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `require'
[...]
/lib/tasks/misc.rake:4
/usr/lib/ruby/1.8/rake.rb:636:in `call'
/usr/lib/ruby/1.8/rake.rb:636:in `execute'
/usr/lib/ruby/1.8/rake.rb:631:in `each'
/usr/lib/ruby/1.8/rake.rb:631:in `execute'
/usr/lib/ruby/1.8/rake.rb:597:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/lib/ruby/1.8/rake.rb:590:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/rake.rb:607:in `invoke_prerequisites'
/usr/lib/ruby/1.8/rake.rb:604:in `each'
/usr/lib/ruby/1.8/rake.rb:604:in `invoke_prerequisites'
/usr/lib/ruby/1.8/rake.rb:596:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/lib/ruby/1.8/rake.rb:590:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/rake.rb:583:in `invoke'
/usr/lib/ruby/1.8/rake.rb:2051:in `invoke_task'
/usr/lib/ruby/1.8/rake.rb:2029:in `top_level'
/usr/lib/ruby/1.8/rake.rb:2029:in `each'
/usr/lib/ruby/1.8/rake.rb:2029:in `top_level'
/usr/lib/ruby/1.8/rake.rb:2068:in `standard_exception_handling'
/usr/lib/ruby/1.8/rake.rb:2023:in `top_level'
/usr/lib/ruby/1.8/rake.rb:2001:in `run'
/usr/lib/ruby/1.8/rake.rb:2068:in `standard_exception_handling'
/usr/lib/ruby/1.8/rake.rb:1998:in `run'
/usr/bin/rake:28
Missing these required gems:
mime-types = 1.17
You're running:
ruby 1.8.7.302 at /usr/bin/ruby1.8
rubygems 1.3.7 at /home/mark/.gem/ruby/1.8, /var/lib/gems/1.8
Run `rake gems:install` to install the missing gems.
However, mime-types was present in vendor/gems/mime-types-1.17. Why doesn't rake think that the gem is installed in this case?
A colleague helpfully fixed this problem for me: it seems that Rails expects all the gems in vendor/gems to have a particular layout, which mime-types doesn't conform to. The solution is to change the line in environment.rb that specifies the dependency on mime-type to add the :lib option, which should specify a relative path that indicates how to get to the source in the gem:
config.gem 'mime-types', :version => '1.17', :lib => 'mime/types'
This means to look in vendor/gems/mime-types-1.17/lib/mime/types rather than the default based on the gem name, vendor/gems/mime-types-1.17/lib/mime-types/