SQLHydra's maxBy causes SqlHydra code to fail if querying an empty table

Noticed this when trying to use a maxBy on an empty table. We were able to work around with a sortByDescending into take 1, (we also could grab a count and branch on whether that returned anything) but wanted to suggest it might be a bit more intuitive to have maxBy handle the null without breaking; perhaps by returning an option?

This problem was encountered using SqlHydra.SqlServer.

I believe this should be reproducible by creating a database with an empty table, creating the schema for said table, and querying for a maxBy on one of the fields. Our particular field was a dateTime2(6) if that is relevant.

[JordanMarr]

let thisWorks = async {
  let! result = selectAsync HydraReader.Read (createContext connString) {
    for record in table<dbo.weather_history > do
    where (record.weather_instrument = 0uy)
    select (maxBy (Some record.entry_timestamp)) 
    tryHead
  }
  return Option.flatten result
}

it’ll work properly because by selecting the max of Some record.entry_timestamp, it makes SqlHydra treat the type as an Option, and it successfully parses DBNull instead of crashing. I wonder if this is worth including in the docs by aggregates? It seems like a fairly common use case where you select an aggregate that might return NULL…

Very creative fix! I think that would definitely be helpful to have in the readme! Also, table names are now generated. See notes for this release.

[ntwilson]

Hey @mjdupontMEA (and anyone else stumbling across this), I discovered a workaround without needing modifications. From your earlier example:

let thisFails = 
  selectAsync HydraReader.Read (createContext connString) {
    for record in table<dbo.weather_history > do
    where (record.weather_instrument = 0uy)
    select (maxBy record.entry_timestamp) }

if you change it to

let thisWorks = async {
  let! result = selectAsync HydraReader.Read (createContext connString) {
    for record in table<dbo.weather_history > do
    where (record.weather_instrument = 0uy)
    select (maxBy (Some record.entry_timestamp)) 
    tryHead
  }
  return Option.flatten result
}

it’ll work properly because by selecting the max of Some record.entry_timestamp, it makes SqlHydra treat the type as an Option, and it successfully parses DBNull instead of crashing. I wonder if this is worth including in the docs by aggregates? It seems like a fairly common use case where you select an aggregate that might return NULL…