Ruby error: no such file to load -- haml (MissingSourceFile)

I am getting this error when I run my project:

C:/InstantRails/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_ori开发者_StackOverflow中文版ginal_require': 
no such file to load -- haml (MissingSourceFile)

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Did you install the haml rubygem?

gem install haml

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You may need to add:

config.gem haml 

to your environment.rb file