elastic search 5.1 why stored_fields does not return asked field?
By default, the document fields are not stored, i.e. in your mapping you don't specify store: true for each of them.
Hence, "stored_fields": ["first_name"] will not be able to return the first_name field since it's not stored.
You can use source filtering instead and specify "_source": ["first_name"] in your query, that will work.