Database.Esqueleto.Record

Documentation

deriveEsqueletoRecord :: Name -> Q [Dec]

Takes the name of a Haskell record type and creates a variant of that record prefixed with Sql which can be used in esqueleto expressions. This reduces the amount of pattern matching on large tuples required to interact with data extracted with esqueleto.

Note that because the input record and the Sql-prefixed record share field names, the {-# LANGUAGE DuplicateRecordFields #-} extension is required in modules that use deriveEsqueletoRecord. Additionally, the {-# LANGUAGE TypeApplications #-} extension is required for some of the generated code.

Given the following record:

data MyRecord = MyRecord
  { myName    :: Text
  , myAge     :: Maybe Int
  , myUser    :: Entity User
  , myAddress :: Maybe (Entity Address)
  }

$(deriveEsqueletoRecord ''MyRecord) will generate roughly the following code:

data SqlMyRecord =
  SqlMyRecord { myName    :: SqlExpr (Value Text)
              , myAge     :: SqlExpr (Value Int)
              , myUser    :: SqlExpr (Entity User)
              , myAddress :: SqlExpr (Maybe (Entity Address))
              }

instance SqlSelect SqlMyRecord MyRecord where
  sqlSelectCols
    identInfo
    SqlMyRecord { myName    = myName
                , myAge     = myAge
                , myUser    = myUser
                , myAddress = myAddress
                } =
    sqlSelectCols identInfo (myName :& myAge :& myUser :& myAddress)

  sqlSelectColCount _ =
    sqlSelectColCount
      (Proxy @(   (SqlExpr (Value Text))
               :& (SqlExpr (Value Int))
               :& (SqlExpr (Entity User))
               :& (SqlExpr (Maybe (Entity Address)))))

  sqlSelectProcessRow columns =
    first ((fromString "Failed to parse MyRecord: ") <>)
          (evalStateT process columns)
    where
      process = do
        Value myName <- takeColumns @(SqlExpr (Value Text))
        Value myAge  <- takeColumns @(SqlExpr (Value Int))
        myUser       <- takeColumns @(SqlExpr (Entity User))
        myAddress    <- takeColumns @(SqlExpr (Maybe (Entity Address)))
        pure MyRecord { myName = myName
                      , myAge = myAge
                      , myUser = myUser
                      , myAddress = myAddress
                      }

Then, we could write a selection function to use the record in queries:

getMyRecord :: SqlPersistT IO [MyRecord]
getMyRecord = select myRecordQuery

myRecordQuery :: SqlQuery SqlMyRecord
myRecordQuery = do
  user :& address <- from $
    table @User
      `leftJoin`
      table @Address
      `on` (do \(user :& address) -> user ^. #address ==. address ?. #id)
  pure
    SqlMyRecord
      { myName = castString $ user ^. #firstName
      , myAge = val 10
      , myUser = user
      , myAddress = address
      }

Since: 3.5.6.0

deriveEsqueletoRecordWith :: DeriveEsqueletoRecordSettings -> Name -> Q [Dec]

Takes the name of a Haskell record type and creates a variant of that record based on the supplied settings which can be used in esqueleto expressions. This reduces the amount of pattern matching on large tuples required to interact with data extracted with esqueleto.

This is a variant of deriveEsqueletoRecord which allows you to avoid the use of {-# LANGUAGE DuplicateRecordFields #-}, by configuring the DeriveEsqueletoRecordSettings used to generate the SQL record.

Since: 3.5.8.0

data DeriveEsqueletoRecordSettings

Codegen settings for deriveEsqueletoRecordWith.

Since: 3.5.8.0

Constructors

DeriveEsqueletoRecordSettings

Fields

• sqlNameModifier :: String -> String

  Function applied to the Haskell record's type name and constructor name to produce the SQL record's type name and constructor name.

  Since: 3.5.8.0

• sqlFieldModifier :: String -> String

  Function applied to the Haskell record's field names to produce the SQL record's field names.

  Since: 3.5.8.0

defaultDeriveEsqueletoRecordSettings :: DeriveEsqueletoRecordSettings

The default codegen settings for deriveEsqueletoRecord.

These defaults will cause you to require {-# LANGUAGE DuplicateRecordFields #-} in certain cases (see deriveEsqueletoRecord.) If you don't want to do this, change the value of sqlFieldModifier so the field names of the generated SQL record different from those of the Haskell record.

Since: 3.5.8.0