Restricting user access based on a site column value in a document library.

Question

 
We have a business requirement to show the contents of a document library based on a value (or values) in the site column (or multiple columns). For example, my document library has a custom site column called confidentiality. This
will have values like restricted, internal and public. Now, based on the AD Group the user belongs to, I should be able to control the access to Restricted or Restricted and Internal files from the document library. We are using SharePoint Online 2010.
Please suggest the best way to achieve this requirement?

Answer 1

SharePoint's security model doesn't allow you to specify security based on metadata. You could however create a Sandboxed Solution containing a Feature that registers a custom event receiver on the Document Library. The logic inside this
Event Receiver would fire after editing item properties (ItemUpdated) to apply item-level permissions based on the rules you need.
Make sure to read the article below to determine if fine-grained permissions are suitable in your case:
http://technet.microsoft.com/en-us/library/gg128955.aspx