I'm building a site in Rails 3.2. Its been 3 years since I've touched Rails or Ruby, so I'm rusty on both, plus the last time I used rails is was Rails 2.3. Needless to say, please excuse any "simple" questions below.

Here are the specs

Multi Tennant CMS/Store Site
- http://company1.mywebsite.com
- http://company2.mywebsite.com
- etc.
Each "Store" (aka sub-domain) can have its own look, feel, etc. through CSS customizations
- The customizations can be performed in a UI within the app allowing the user to change basic variables of Bootstrap (i.e. @textColor, @bodyBackground, etc.)
I'm using the less-rails-bootstrap gem to the Twitter Bootstrap look/feel, etc.

Here are the challenges

I need to be able to dynamically output the variables for the CSS into a file that gets mixed in to Bootstrap so the variables are picked up to create the final CSS
When a user changes a variable for the CSS, the existing style is basically invalidated. I need the full CSS recompiled and written back out to disk, memory stream, or some other location where I can get my hands on it (remember this is using less)
I need different CSS to spit out per sub-domain. Any suggestions on how to approach this?

Further complicating the matter...

...given that I essentially will have to find some way to compile the CSS on the fly, that means I have to include GEMS I typically would not in a production environment. Performance will be very important. Is there a way to isolate this? Once the CSS has been invalidated and regenerated, I could take the content and either write it out to disk or store is in some memcached/redis/etc. instance for performance.

Any comments, even if just to point me in a general direction would be appreciated.

Thanks!

Here is the solution I finally landed on:

I ended up switching over to bootstrap-sass instead https://github.com/thomas-mcdonald/bootstrap-sass

Made the following changes to my application.rb file to ensure that the :asset group is always included despite the environment:

if defined?(Bundler)
    # If you precompile assets before deploying to production, use this line
    # Bundler.require(*Rails.groups(:assets => %w(development test)))
    # If you want your assets lazily compiled in production, use this line
    Bundler.require(:default, :assets, Rails.env)
end

Used the concepts provided by Manuel Meure (Thank you Manuel!) of Kraut Computing found at http://www.krautcomputing.com/blog/2012/03/27/how-to-compile-custom-sass-stylesheets-dynamically-during-runtime/ .
- I made some adjustments to suit my own needs, but the core concepts illustrated by Manuel were the foundation for my compilation process.

In my model (lets call it "Site"), I have a snippet of code that looks like this:

# .../app/models/site.rb
...

BASE_STYLE = "
  @import \"compass/css3\";

  <ADDITIONAL_STYLES>

  @import \"bootstrap\";
  @import \"bootstrap-responsive\";
".freeze

# Provides the SASS/CSS content that would 
# be included into your base SASS content before compilation
def sass_content
  "
  $bodyBackground: #{self.body_background};
  $textColor: #{self.text_color};
  " + self.css # Any additional CSS/SASS you would want to add
end

def compile_css(test_only = false, force_recompile = false)

  # SassCompiler is a modification of the information made available at the Kraut Computing link
  compiler = SassCompiler.new("#{self.id}/site.css", {:syntax => :scss, :output_dir => Rails.root.join('app', 'assets', 'sites')})

  # Bail if we're already compiled and we're not forcing recompile
  return if compiler.compiled? && !force_recompile && !test_only

  # The block here yields the content that will be rendered
  compiler.compile(test_only) {
    # take our base styles, slap in there some vars that we make available to be customized by the user
    # and then finally add in our css/scss that the user updated... concat those and use it as
    # our raw sass to compile
    BASE_STYLE.gsub(/<ADDITIONAL_STYLES>/, self.sass_content)
  }
end

I hope this helps. I know its a deviation from the original post, but its deviated because this seemed to be the most attainable solution to the problem.

If I haven't answered a specific question you have, feel free to comment so I can expand where possible.

Thanks!