How to get the id of a database entity with Persistent?

Question

I have a database model, using Persistent like so

import           Database.Persist.TH (mkPersist, persistUpperCase,
                                  share, sqlSettings)

share [mkPersist sqlSettings] [persistUpperCase|
Foo
    field1 Int
    field2 Bool
|]

And I am able to get an object foo :: Foo from the database. I can access the fields with fooField1 foo :: Int and fooField2 foo :: Bool. And because I use sqlSettings, I know there is an Int64-representation of a database key the "id" stored with every entity. E.g. when I use get . toSqlKey :: Int64 -> ...

Given my foo :: Foo, how do I get the id :: Int64?

Answer 1

A Foo by itself doesn't have an id, since Foos can exist outside of the database (before they are written). That's why a select operation for example gives you back a list of Entity, that contains both the key and the object.

See http://hackage.haskell.org/package/persistent-2.2/docs/Database-Persist-Types.html#t:Entity

For example, see the yesod book (http://www.yesodweb.com/book/persistent) on fetching from a primary key constraint:

personId <- insert $ Person "Michael" "Snoyman" 26
maybePerson <- getBy $ PersonName "Michael" "Snoyman"
case maybePerson of
    Nothing -> liftIO $ putStrLn "Just kidding, not really there"
    Just (Entity personId person) -> liftIO $ print person

the return type of getBy is an Entity wrapping both the key (personId) and the value (person).