Fix issue where column couldn't be altered multiple times

Previously, having multiple alter_column actions for a single column
could cause issues as the triggers and batch updates referenced the
wrong columns.

This commit also simplifies the batch update procedure. Rather than
running the actual update on existing rows, a NOP update will be
run which in turn will trigger the triggers to update the new temporary
columns.

src/migrations/add_column.rs

@@ -134,8 +134,8 @@ impl Action for AddColumn {
         }
 
         // Backfill values in batches
-        if let Some(up) = &self.up {
-            common::batch_update(db, table, &self.column.name, up)?;
+        if self.up.is_some() {
+            common::batch_touch_rows(db, table, &self.column.name)?;
         }
 
         Ok(())

src/migrations/alter_column.rs

@@ -154,7 +154,7 @@ impl Action for AlterColumn {
             RETURNS TRIGGER AS $$
             DECLARE
                 {declarations}
-                {existing_column} public.{table}.{existing_column}%TYPE := NEW.{existing_column};
+                {existing_column} public.{table}.{existing_column}%TYPE := NEW.{existing_column_real};
             BEGIN
                 NEW.{temp_column} = {up};
                 RETURN NEW;
@@ -180,6 +180,7 @@ impl Action for AlterColumn {
             CREATE TRIGGER {update_new_trigger} BEFORE UPDATE OF {temp_column} ON {table} FOR EACH ROW EXECUTE PROCEDURE {update_new_trigger}();
             ",
             existing_column = column.name,
+            existing_column_real = column.real_name(),
             temp_column = self.temporary_column_name(ctx),
             up = up,
             down = down,
@@ -191,6 +192,9 @@ impl Action for AlterColumn {
         );
         db.run(&query)?;
 
+        // Backfill values in batches
+        common::batch_touch_rows(db, table, &column.name)?;
+
         // Add a temporary NOT NULL constraint if the column shouldn't be nullable.
         // This constraint is set as NOT VALID so it doesn't apply to existing rows and
         // the existing rows don't need to be scanned under an exclusive lock.
@@ -209,9 +213,6 @@ impl Action for AlterColumn {
             db.run(&query)?;
         }
 
-        // Backfill values in batches
-        common::batch_update(db, table, &self.temporary_column_name(ctx), up)?;
-
         Ok(())
     }
 

src/migrations/common.rs

@@ -83,14 +83,9 @@ impl ToSql for PostgresRawValue {
     postgres::types::to_sql_checked!();
 }
 
+pub fn batch_touch_rows(db: &mut dyn Conn, table: &Table, column: &str) -> anyhow::Result<()> {
     const BATCH_SIZE: u16 = 1000;
 
-pub fn batch_update(
-    db: &mut dyn Conn,
-    table: &Table,
-    column: &str,
-    up: &str,
-) -> anyhow::Result<()> {
     let mut cursor: Option<PostgresRawValue> = None;
 
     loop {
@@ -139,7 +134,7 @@ pub fn batch_update(
                     LIMIT {batch_size}
                 ), update AS (
                     UPDATE {table}
-                    SET {temp_column} = {up}
+                    SET {column} = {column}
                     FROM rows
                     WHERE {primary_key_where}
                     RETURNING {returning_columns}
@@ -152,8 +147,7 @@ pub fn batch_update(
             primary_key_columns = primary_key_columns,
             cursor_where = cursor_where,
             batch_size = BATCH_SIZE,
-            temp_column = column,
-            up = up,
+            column = column,
             primary_key_where = primary_key_where,
             returning_columns = returning_columns,
         );
@@ -165,9 +159,9 @@ pub fn batch_update(
         if last_value.is_none() {
             break;
         }
-        println!("{}", query);
 
         cursor = last_value
     }
+
     Ok(())
 }

tests/alter_column.rs

@@ -291,3 +291,98 @@ fn alter_column_rename() {
 
     reshape.complete_migration().unwrap();
 }
+
+#[test]
+fn alter_column_multiple() {
+    let (mut reshape, mut old_db, mut new_db) = common::setup();
+
+    let create_users_table = Migration::new("create_users_table", None).with_action(CreateTable {
+        name: "users".to_string(),
+        primary_key: vec!["id".to_string()],
+        foreign_keys: vec![],
+        columns: vec![
+            Column {
+                name: "id".to_string(),
+                data_type: "SERIAL".to_string(),
+                nullable: true,
+                default: None,
+            },
+            Column {
+                name: "counter".to_string(),
+                data_type: "INTEGER".to_string(),
+                nullable: false,
+                default: None,
+            },
+        ],
+    });
+    let increment_counter_twice = Migration::new("increment_counter_twice", None)
+        .with_action(AlterColumn {
+            table: "users".to_string(),
+            column: "counter".to_string(),
+            up: Some("counter + 1".to_string()),
+            down: Some("counter - 1".to_string()),
+            changes: ColumnChanges {
+                data_type: None,
+                nullable: None,
+                name: None,
+            },
+        })
+        .with_action(AlterColumn {
+            table: "users".to_string(),
+            column: "counter".to_string(),
+            up: Some("counter + 1".to_string()),
+            down: Some("counter - 1".to_string()),
+            changes: ColumnChanges {
+                data_type: None,
+                nullable: None,
+                name: None,
+            },
+        });
+
+    let first_migrations = vec![create_users_table.clone()];
+    let second_migrations = vec![create_users_table.clone(), increment_counter_twice.clone()];
+
+    // Run first migration, should automatically finish
+    reshape.migrate(first_migrations.clone()).unwrap();
+    assert!(matches!(reshape.state.status, Status::Idle));
+    assert_eq!(
+        Some(&create_users_table.name),
+        reshape.state.current_migration.as_ref()
+    );
+
+    // Update search paths
+    old_db
+        .simple_query(&reshape::schema_query_for_migration(
+            &first_migrations.last().unwrap().name,
+        ))
+        .unwrap();
+
+    // Insert some test data
+    old_db
+        .simple_query(
+            "
+            INSERT INTO users (id, counter) VALUES
+                (1, 0),
+                (2, 100);
+            ",
+        )
+        .unwrap();
+
+    // Run second migration
+    reshape.migrate(second_migrations.clone()).unwrap();
+    new_db
+        .simple_query(&reshape::schema_query_for_migration(
+            &second_migrations.last().unwrap().name,
+        ))
+        .unwrap();
+
+    // Check that the existing data has been updated
+    let expected = vec![2, 102];
+    let results: Vec<i32> = new_db
+        .query("SELECT counter FROM users ORDER BY id", &[])
+        .unwrap()
+        .iter()
+        .map(|row| row.get::<_, i32>("counter"))
+        .collect();
+    assert_eq!(expected, results);
+}