fix: simple ORM for one struct and foreign references work

Currently, all methods declared in the Georm trait are available.

If a struct has an ID pointing towards another entity, the user can
create a get method to get the entity pointed at from the database
too (local one-to-one relationship).

I still need to implement remote one-to-one relationships (one-to-one
relationships when the ID of the remote object is not available
locally).

I still need to also test and debug one-to-many relationships (ID of
the remote entiies not available locally) and many-to-many
relationships (declared in a dedicated table).

For now, IDs in all cases are simple types recognized by SQLx that are
not arrays. Options are only supported when explicitely specified for
one-to-one relationships.

src/lib.rs

@@ -1,6 +1,16 @@
 pub use georm_macros::Georm;
 
 pub trait Georm<Id> {
+    /// Find all the entities in the database.
+    ///
+    /// # Errors
+    /// Returns any error Postgres may have encountered
+    fn find_all(
+        pool: &sqlx::PgPool,
+    ) -> impl ::std::future::Future<Output = ::sqlx::Result<Vec<Self>>> + Send
+    where
+        Self: Sized;
+
     /// Find the entiy in the database based on its identifier.
     ///
     /// # Errors
@@ -42,9 +52,18 @@ pub trait Georm<Id> {
     fn create_or_update(
         &self,
         pool: &sqlx::PgPool,
-    ) -> impl std::future::Future<Output = sqlx::Result<Self>> + Send
+    ) -> impl ::std::future::Future<Output = sqlx::Result<Self>>
     where
-        Self: Sized;
+        Self: Sized,
+    {
+        async {
+            if Self::find(pool, self.get_id()).await?.is_some() {
+                self.update(pool).await
+            } else {
+                self.create(pool).await
+            }
+        }
+    }
 
     /// Delete the entity from the database if it exists.
     ///