In that scenario, you can enable Pega default Class key for that particular Data Class. To configure that,
Open the Class record for the class associated to the Data Type
Enable Automatically generate a unique ID for records of this type
After saving the class, if you try to configure the Source from the Data Type, the property pyGUID will be enabled as key by default. The Pega behind generate a random value for this property while creating new instances in this class.
I hope it helps. Let us know if you face any other issues