There is actually no change in creating a WCF RIA Service in Lightswitch 2012, just follow the guide I did in Lightswitch 2011 (How to create a VB WCF RIA Services for Microsoft Lightswitch to aggregate or join tables at the same time pass a parameter?).
Just be very careful on the framework version. As seen in the image below, by double clicking on the My Project item of the WCF RIA Project you can open the project setup which contains the project references. The version should be 4.0 not 4.5.
The default framework run time version when creating a Visual Studio 2012 project is 4.5, to change just go to the Application tab and change the target framework as shown in the image below.
Thats it, just rebuild the WCF RIA Service project and its ready for Lightswitch 2012 consumption.
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