Hi!
Asked the same question some time ago in this post
Is there any chance that the Etag can be treated as key? (i.e. is already hashed version of the self link) or is better safer to use the self link for that.
And the answer was
The etag cannot be treated as a key. The plan is for the etag to be used by customers as a way of returning data only if its changed since the last time they requested the resource, but we have yet to implement the functionality. The following forum post discusses in more detail. Etags in documentation, but not in resources - #2 by h_brackmann.
Please use the self link as the key as it is the unique identifier for the resource.
Have a lovely day,
Patryk