Get items with selectList and filters. when field is Maybe

[Conee]

Hi,

I have data :

Address

   state Text sqltype=varchar(255)

    city Text sqltype=varchar(255)

    street Text sqltype=varchar(255)

    number Text sqltype=varchar(255)

    deriving Show Typeable Eq

Location

   name Text sqltype=varchar(255)

   address AddressId sqltype=varchar(255)

   UniqueLocationName name

   deriving Show Typeable

Manifestation

  name Text sqltype=varchar(255)

   description Text Maybe sqltype=varchar(255)

   category Category Maybe

   startDateTime UTCTime sqltype=DateTime

   location LocationId sqltype=varchar(255)

  UniqueManName name

   deriving Show Typeable

data Category = Sport | Concert | Theater

   deriving (Show, Eq, Read, Enum, Bounded)

   derivePersistField "Category"

What I want to do is to filter Manifestations, with the data I receive from users. Data for filtering is : [Category], city :: Text and search :: Text.

I imagined doing this with a selectList and its ability to filter items from the database but here I come up with one problem, so I would ask for help if anyone has an idea.

1. The first mistake is that my ManifestationCategory field is in the Maybe wrapper and I don't know way to handle this:

filterManifestations :: [Category] -> Maybe T.Text -> Maybe T.Text -> Handler [Entity Manifestation]

filterManifestations cat city src = do

    let city' = fromMaybe city

       src' = fromMaybe src

    runDB $ selectList [ManifestationCategory <-. cat] []

  -- Couldn't match type  ‘Category’     with ‘Maybe Category’
    arising from a use of ‘ManifestationCategory’
    • In the first argument of ‘(<-.)’, namely ‘ManifestationCategory’

Of course any advice is welcome,

thanks,

Nemanja

[jsch...@gmail.com]

Hi,

I don't know the (<-.) operator. But if I want to selectList with a filter, I use the (==.) operator for equality check.

- Jakob