ForeignKeyField (null=True, lazy_load=False) whose instance value is the related row's id, typed str | None.

With lazy_load=False peewee never loads the related row: instance access returns the raw foreign-key id (a str, or None when unset) instead of the related model. Use this when you only ever need the id and want to avoid the extra query:

source = NullableForeignKeyIdField(ResourceModel)   # -> str | None

For a self-reference, pass "self" (no generic to specialize, the value is always str | None):

parent = NullableForeignKeyIdField("self")   # -> str | None