Many to Many Documents App
This app demonstrates how you can leverage a many to many relationship to share a single document with multiple projects.
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This "many to many" technique can be used anytime you don't have a simple one to many relationship structure. The "link" table just serves as the join between Projects and Documents.
This app was really designed just as a feature example, however you could probably take it an use it as a very simple Document Storage repository right out of the box. Enjoy!