$expand Query not working

Hello Experts,

I am facing issue with $expand.

I have followed below steps,

1. Create Data Mode - SalesOrder and OrderItems

2. Association - SalesOrderItems between above two DataModels,

3.Implemented method - /IWBEP/IF_MGW_APPL_SRV_RUNTIME~GET_EXPANDED_ENTITY

When I am executing URI i am getting error "Method 'ORDERITEMS_GET_ENTITYSET' not implemented in data provider class."

As per my understanding /IWBEP/IF_MGW_APPL_SRV_RUNTIME~GET_EXPANDED_ENTITY method alone should work.

My URI - /sap/opu/odata/sap/ZSL_EMP_DEMO1212_SRV/SalesOrder('500000000')?$expand=SalesOrderItems

Can anyone please help me if I am missing anything here.

Regards,

Sid