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Module: Subscriptions by Reference
What does it do?
The Subscriptions module allows you to subscribe directly to nodes, but what if you want to subscribe to one node referenced by another? Previously, you could do that with a moderate amount of custom code, maybe with the help of a blog post explaining the process. Now, the Subscriptions by Reference module promises to make that even easier. Just tell it which content types and field you want to use in the subscription and it does the rest.
Look Useful? Review it!
If you would like to see this module readily available on Drupal.org, you should review it and help make that happen.