When using SASS how can I import a file from a different directory?

UPDATE: Please consider Mikka's answer first - it should work without flaw if you're only interested in including subdirectories. My answer is specific to including directories other than subdirectories, but works for those, too. But: It's not the idiomatic way to do it.

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Looks like some changes to SASS have made possible what you've initially tried doing:

@import "../subdir/common";

We even got this to work for some totally unrelated folder located in c:\projects\sass:

@import "../../../../../../../../../../projects/sass/common";

Just add enough ../ to be sure you'll end up at the drive root and you're good to go.

Of course, this solution is far from pretty, but I couldn't get an import from a totally different folder to work, neither using I c:\projects\sass nor setting the environment variable SASS_PATH (from: :load_paths reference) to that same value.

You could use the -I command line switch or :load_paths option from Ruby code to add sub_directory_a to Sass's load path. So if you're running Sass from root_directory, do something like this:

sass -I sub_directory_a --watch sub_directory_b:sub_directory_b

Then you can simply use @import "common" in more_styles.scss.