fix and extend postgres transaction example (#1636)

Author: taladar

examples/postgres/transaction/src/main.rs

@@ -1,37 +1,104 @@
 use sqlx::query;
 
-#[async_std::main]
-async fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let conn_str =
-        std::env::var("DATABASE_URL").expect("Env var DATABASE_URL is required for this example.");
-    let pool = sqlx::PgPool::connect(&conn_str).await?;
-
-    let mut transaction = pool.begin().await?;
-
-    let test_id = 1;
+async fn insert_and_verify(
+    transaction: &mut sqlx::Transaction<'_, sqlx::Postgres>,
+    test_id: i64,
+) -> Result<(), Box<dyn std::error::Error>> {
     query!(
         r#"INSERT INTO todos (id, description)
         VALUES ( $1, $2 )
         "#,
         test_id,
         "test todo"
     )
-    .execute(&mut transaction)
+    .execute(&mut *transaction)
     .await?;
 
-    // check that inserted todo can be fetched
+    // check that inserted todo can be fetched inside the uncommitted transaction
     let _ = query!(r#"SELECT FROM todos WHERE id = $1"#, test_id)
-        .fetch_one(&mut transaction)
+        .fetch_one(transaction)
         .await?;
 
-    transaction.rollback();
+    Ok(())
+}
+
+async fn explicit_rollback_example(
+    pool: &sqlx::PgPool,
+    test_id: i64,
+) -> Result<(), Box<dyn std::error::Error>> {
+    let mut transaction = pool.begin().await?;
+
+    insert_and_verify(&mut transaction, test_id).await?;
+
+    transaction.rollback().await?;
+
+    Ok(())
+}
+
+async fn implicit_rollback_example(
+    pool: &sqlx::PgPool,
+    test_id: i64,
+) -> Result<(), Box<dyn std::error::Error>> {
+    let mut transaction = pool.begin().await?;
+
+    insert_and_verify(&mut transaction, test_id).await?;
+
+    // no explicit rollback here but the transaction object is dropped at the end of the scope
+    Ok(())
+}
+
+async fn commit_example(
+    pool: &sqlx::PgPool,
+    test_id: i64,
+) -> Result<(), Box<dyn std::error::Error>> {
+    let mut transaction = pool.begin().await?;
+
+    insert_and_verify(&mut transaction, test_id).await?;
+
+    transaction.commit().await?;
 
-    // check that inserted todo is now gone
+    Ok(())
+}
+
+#[async_std::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let conn_str =
+        std::env::var("DATABASE_URL").expect("Env var DATABASE_URL is required for this example.");
+    let pool = sqlx::PgPool::connect(&conn_str).await?;
+
+    let test_id = 1;
+
+    // remove any old values that might be in the table already with this id from a previous run
+    let _ = query!(r#"DELETE FROM todos WHERE id = $1"#, test_id)
+        .execute(&pool)
+        .await?;
+
+    explicit_rollback_example(&pool, test_id).await?;
+
+    // check that inserted todo is not visible outside the transaction after explicit rollback
     let inserted_todo = query!(r#"SELECT FROM todos WHERE id = $1"#, test_id)
         .fetch_one(&pool)
         .await;
 
     assert!(inserted_todo.is_err());
 
+    implicit_rollback_example(&pool, test_id).await?;
+
+    // check that inserted todo is not visible outside the transaction after implicit rollback
+    let inserted_todo = query!(r#"SELECT FROM todos WHERE id = $1"#, test_id)
+        .fetch_one(&pool)
+        .await;
+
+    assert!(inserted_todo.is_err());
+
+    commit_example(&pool, test_id).await?;
+
+    // check that inserted todo is visible outside the transaction after commit
+    let inserted_todo = query!(r#"SELECT FROM todos WHERE id = $1"#, test_id)
+        .fetch_one(&pool)
+        .await;
+
+    assert!(inserted_todo.is_ok());
+
     Ok(())
 }