Database.Persist.ImplicitIdDef

A specification for how the implied ID columns are created.

By default, persistent will give each table a default column named id (customizable by PersistSettings), and the column type will be whatever you'd expect from BackendKey yourBackendType. For The SqlBackend type, this is an auto incrementing integer primary key.

You might want to give a different example. A common use case in postgresql is to use the UUID type, and automatically generate them using a SQL function.

Previously, you'd need to add a custom Id annotation for each model.

User
    Id   UUID default="uuid_generate_v1mc()"
    name Text

Dog
    Id   UUID default="uuid_generate_v1mc()"
    name Text
    user UserId

Now, you can simply create an ImplicitIdDef that corresponds to this declaration.

newtype UUID = UUID ByteString

instance PersistField UUID where
    toPersistValue (UUID bs) =
        PersistLiteral_ Escaped bs
    fromPersistValue pv =
        case pv of
            PersistLiteral_ Escaped bs ->
                Right (UUID bs)
            _ ->
                Left "nope"

instance PersistFieldSql UUID where
    sqlType _ = SqlOther UUID

With this instance at the ready, we can now create our implicit definition:

uuidDef :: ImplicitIdDef
uuidDef = mkImplicitIdDef @UUID "uuid_generate_v1mc()"

And we can use setImplicitIdDef to use this with the MkPersistSettings for our block.

mkPersist (setImplicitIdDef uuidDef sqlSettings) [persistLowerCase| ... |]

TODO: either explain interaction with mkMigrate or fix it. see issue #1249 for more details.

Create an ImplicitIdDef based on the Typeable and PersistFieldSql constraints in scope.

This function uses the TypeApplications syntax. Let's look at an example that works with Postgres UUIDs.

newtype UUID = UUID Text
    deriving newtype PersistField

instance PersistFieldSql UUID where
    sqlType _ = SqlOther "UUID"

idDef :: ImplicitIdDef
idDef = mkImplicitIdDefTypeable @UUID "uuid_generate_v1mc()"

This ImplicitIdDef will generate default UUID columns, and the database will call the uuid_generate_v1mc() function to generate the value for new rows being inserted.

If the type t is Text or String then a max_len attribute of 200 is set. To customize this, use setImplicitIdDefMaxLen.

This is the default variant. Setting the implicit ID definition to this value should not have any change at all on how entities are defined by default.

Set the maximum length of the implied ID column. This is required for any type where the associated SqlType is a TEXT or VARCHAR sort of thing.

Remove the default attribute of the ImplicitIdDef column. This will require you to provide an ID for the model with every insert, using insertKey instead of insert, unless the type has some means of getting around that in the migrations.

As an example, the Postgresql SERIAL type expands to an autoincrementing integer. Postgres will implicitly create the relevant series and set the default to be NEXTVAL(series_name). A default is therefore unnecessary to use for this type.

However, for a UUID, postgres *does not* have an implicit default. You must either specify a default UUID generation function, or insert them yourself (again, using insertKey).

This function will be deprecated in the future when omiting the default implicit ID column is more fully supported.