$similarity accepted within projection

[hemidactylus]

Dollar-fields raise an UNSUPPORTED_PROJECTION_PARAM error all right, while (perhaps unexpected since there's the includeSimilarity option already to control that) as special case "$similarity": 1 is accepted by the find methods.

ERROR:

curl -XPOST   "${ASTRA_DB_API_ENDPOINT}/api/json/v1/default_keyspace/astra_db_quickstart"   -H "X-Cassandra-Token: $ASTRA_DB_APPLICATION_TOKEN"   -H "Content-Type: application/json"   -d '{
    "findOne": {
      "sort" : {"$vector" : [0.15, 0.1, 0.1, 0.35, 0.55]},
      "projection": {"text": 1, "$similarity": 1, "$pizza": 1},
      "options" : {
        "includeSimilarity" : true
      }
    }
  }' | jq

NO ERRORS (just take out the , "$pizza": 1 bit):

curl -XPOST   "${ASTRA_DB_API_ENDPOINT}/api/json/v1/default_keyspace/astra_db_quickstart"   -H "X-Cassandra-Token: $ASTRA_DB_APPLICATION_TOKEN"   -H "Content-Type: application/json"   -d '{
    "findOne": {
      "sort" : {"$vector" : [0.15, 0.1, 0.1, 0.35, 0.55]},
      "projection": {"text": 1, "$similarity": 1},
      "options" : {
        "includeSimilarity" : true
      }
    }
  }' | jq

[tatu-at-datastax]

Thank you for reporting this @hemidactylus.

This does look inconsistent and redundant: while we have some exceptions already ($vector and $vectorize pseudo-fields) this case looks like we should only allow inclusion via includeSimilarity.
I will discuss this with team to know whether behavior needs to be documented or changed.

[vkulichenko]

+1. $similarity in projection should result in error. It's not part of a document (unlike $vector), so it doesn't belong there.

[tatu-at-datastax]

@vkulichenko That makes sense.