What is the best way approach to attach workflow and Event Receiver on External List? How to store Directly External Data to SharePoint Custom List?List
As I know the external list has about 25 limitations, because the data is stored into external data base. In your case you faced with these limitations:
In this post you can read
but I have not find any examples. I found out that in SharePoint 2013 you can use app to solve this issue. See this post for more details.