feat: add foreign one_to_one relationships

tests/fixtures/simple_struct.sql

@@ -1,6 +1,7 @@
 INSERT INTO biographies (content)
 VALUES ('Some text'),
-       ('Some other text');
+       ('Some other text'),
+       ('Biography for no one');
 
 INSERT INTO authors (name, biography_id)
 VALUES ('J.R.R. Tolkien', 2),

tests/models.rs

@@ -1,7 +1,12 @@
 use georm::Georm;
 
 #[derive(Debug, sqlx::FromRow, Georm, PartialEq, Eq, Default)]
-#[georm(table = "biographies")]
+#[georm(
+    table = "biographies",
+    one_to_one = [{
+        name = "author", remote_id = "biography_id", table = "authors", entity = Author
+    }]
+)]
 pub struct Biography {
     #[georm(id)]
     pub id: i32,

tests/o2o_relationship.rs

@@ -53,3 +53,24 @@ async fn books_are_found_despite_nonstandard_id_name(pool: sqlx::PgPool) -> sqlx
     assert_eq!(tolkien, book.get_author(&pool).await?);
     Ok(())
 }
+
+#[sqlx::test(fixtures("simple_struct"))]
+async fn biographies_should_find_remote_o2o_author(pool: sqlx::PgPool) -> sqlx::Result<()> {
+    let london = Author::find(&pool, &3).await?.unwrap();
+    let london_biography = Biography::find(&pool, &1).await?.unwrap();
+    let result = london_biography.get_author(&pool).await;
+    assert!(result.is_ok());
+    let result = result.unwrap();
+    assert!(result.is_some());
+    let result = result.unwrap();
+    assert_eq!(london, result);
+    Ok(())
+}
+
+#[sqlx::test(fixtures("simple_struct"))]
+async fn biographies_may_not_have_corresponding_author(pool: sqlx::PgPool) -> sqlx::Result<()> {
+    let biography = Biography::find(&pool, &3).await?.unwrap();
+    let result = biography.get_author(&pool).await?;
+    assert!(result.is_none());
+    Ok(())
+}