The _id value should be set in the document by the shell (driver) before it is sent to the server.

> var test =

{a:1}

> db.test11.insert(test)
> test

{ "a" : 1 }

— I would like to see the _id field after the insert

> var test =

{a:1}

> db.test11.insert(test)
> test

{ "_id" : ObjectId("...."), "a" : 1 }