Expressions.hs

{-# LANGUAGE FlexibleContexts  #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RecordWildCards   #-}
{-# LANGUAGE TemplateHaskell   #-}
{-# LANGUAGE TypeFamilies      #-}


{-|

This module contains RethinkDB Expressions which are used to build queries.

-}

module Avers.Storage.Expressions where


import           Prelude hiding (lookup)

import           Data.Text (Text)
import           Database.RethinkDB as R


-- | The primary key in all our documents is the default "id".
primaryKeyField :: Text
primaryKeyField = "id"


-- | Expression which represents the primary key field.
primaryKeyFieldE :: Exp Text
primaryKeyFieldE = lift primaryKeyField


-- | Expression which represents the value of a field inside of an Object.
objectFieldE :: (IsDatum a) => Text -> Exp Object -> Exp a
objectFieldE field obj = GetField (lift field) obj


-- | True if the object field matches the given value.
objectFieldEqE :: (ToDatum a) => Text -> a -> Exp Object -> Exp Bool
objectFieldEqE field value obj = Eq
    (objectFieldE field obj :: Exp Datum)
    (lift $ toDatum value)


-- | True if the object's primary key matches the given string.
primaryKeyEqE :: Text -> Exp Object -> Exp Bool
primaryKeyEqE = objectFieldEqE primaryKeyField


-- | Take the first item out of a sequence.
headE :: (IsSequence a, IsDatum r) => Exp a -> Exp r
headE = Nth 0