When doing a single-record or batch read through a client or aql using the digest, the primary key is not returned even though the record was written with sendKey=true, as_policy_key=AS_POLICY_KEY_SEND, or equivalent.
Answer
send_key was implemented to allow confirmation that the key matches on a read (if the primary key is sent as well as the digest on a read, it will return an error if they don't match) to guard against the already infinitesimal chance of a RIPE-MD160 collision. The stored primary key is better considered as a checksum that happens to be the same bytes as the primary key generated by the client. To be able to retrieve the primary key from a record without knowing that key in advance, the key needs to be explicitly stored in a bin.
Notes
Primary and secondary index queries do return the stored key for checking by the client application, as primary keys cannot be sent for those transactions.
