add get_id and relationships, untested

gejdr-core/src/models/mod.rs

@@ -70,4 +70,7 @@ pub trait Crud<Id> {
         pool: &sqlx::PgPool,
         id: &Id,
     ) -> impl std::future::Future<Output = sqlx::Result<u64>> + Send;
+
+    /// Returns the identifier of the entity.
+    fn get_id(&self) -> &Id;
 }