How to get selected key value in sap.m.Select?

Hi Gurus,

I bound the items with a entity set in my OData, but now here comes the question, i.e. how could I get the use selected key or value?

Here is the select/dropdown box snapshot.

And here is the code snippet snapshot.

The content in BPRoleModel>/d/results, as what is seen in the last snap pic, has been populated in the select/dropdown box successfully via this in controller.

The value BPGroup is a field name in an entity.

How could I get the key value and put it into the field BPGroup?