Thanks Jon for your prompt response. Really appreciate it.
I tried using this expression for nms:delivery schema. However, it gave me this error:
25/02/2019 12:15:33 QUE-370015 The node of path '/contentDef/source' is not stored as an independent SQL field (document of type 'Deliveries (nms:delivery)'). Unable to use it as a filtering criteria in a query.