MonadBeam...Returning should be generalized #775
Description
Currently we have:
class MonadBeam be m => MonadBeamDeleteReturning be m | m -> be where
runDeleteReturningList
:: ( Beamable table
, Projectible be (table (QExpr be ()))
, FromBackendRow be (table Identity) )
=> SqlDelete be table
-> m [table Identity]
class MonadBeam be m => MonadBeamInsertReturning be m | m -> be where
runInsertReturningList
:: ( Beamable table
, Projectible be (table (QExpr be ()))
, FromBackendRow be (table Identity) )
=> SqlInsert be table
-> m [table Identity]
class MonadBeam be m => MonadBeamUpdateReturning be m | m -> be where
runUpdateReturningList
:: ( Beamable table
, Projectible be (table (QExpr be ()))
, FromBackendRow be (table Identity) )
=> SqlUpdate be table
-> m [table Identity]The limitation of these is that they all return the full rows, while one might only want a subset of the rows.
For this reason, the Postgres backend provides the additional functions runPg...ReturningList functions, where we see type signatures such as:
deleteReturning :: Projectible Postgres a
=> DatabaseEntity Postgres be (TableEntity table)
-> (forall s. table (QExpr Postgres s) -> QExpr Postgres s Bool)
-> (table (QExpr Postgres PostgresInaccessible) -> a)
-> PgDeleteReturning (QExprToIdentity a)The key difference is that instead of returning table Identity, we pass in a projection function, of type (table (QExpr Postgres PostgresInaccessible) -> a), and we use it to project out the fields we want to return (instead of returning the whole row). So we're replacing table Identity with QExprToIdentity a, where a is the chosen row type to project out.
Given that the SQLite backend supports this feature as well, it seems natural to generalise these monads.