Here are some test cases which use deref. 

• jstests/core/query/dbref/ref4.js
• jstests/core/query/dbref/ref.js
• jstests/core/query/dbref/dbref1.js
• jstests/core/query/dbref/dbref3.js
   
  A  DBRef field has ordered field ($id, $ref, [$db]). DBRefs are references from one document to another using the value of the first document's _id field, collection name, and, optionally, its database name, as well as any other fields. DBRefs allow you to more easily reference documents stored in multiple collections or databases.
  In this ticket, we need to investigate how to support dbref in multitenancy environment. Does mongod need to serialize/deserialize the "$db" with serialization context.