Check for new schema rather than old in helper

We want it to be possible for Reshape to be adopted for existing
databases with an existing schema. To make this possible, we can't check
if the schema path matches the current migration as there won't be a
current migration at first. Instead we'll compare against the target
migration.

The next step in making Reshape work with existing databases is to not
automatically complete the first migration. This will not be safe for an
existing application adopting Reshape.

src/helpers.rs

@@ -2,37 +2,28 @@ use anyhow::Context;
 
 use crate::db::Conn;
 
-pub fn set_up_helpers(db: &mut dyn Conn, current_migration: &Option<String>) -> anyhow::Result<()> {
-    let predicate = if let Some(current_migration) = current_migration {
-        format!(
-            "current_setting('search_path') = 'migration_{}' OR setting_bool",
-            current_migration
-        )
-    } else {
-        "setting_bool".to_string()
-    };
-
+pub fn set_up_helpers(db: &mut dyn Conn, target_migration: &str) -> anyhow::Result<()> {
     let query = format!(
         "
-			CREATE OR REPLACE FUNCTION reshape.is_old_schema()
+			CREATE OR REPLACE FUNCTION reshape.is_new_schema()
 			RETURNS BOOLEAN AS $$
             DECLARE
-                setting TEXT := current_setting('reshape.is_old_schema', TRUE);
+                setting TEXT := current_setting('reshape.is_new_schema', TRUE);
                 setting_bool BOOLEAN := setting IS NOT NULL AND setting = 'YES';
 			BEGIN
-				RETURN {};
+				RETURN current_setting('search_path') = 'migration_{}' OR setting_bool;
 			END
 			$$ language 'plpgsql';
         ",
-        predicate
+        target_migration,
     );
     db.query(&query)
-        .context("failed creating helper function reshape.is_old_schema()")?;
+        .context("failed creating helper function reshape.is_new_schema()")?;
 
     Ok(())
 }
 
 pub fn tear_down_helpers(db: &mut dyn Conn) -> anyhow::Result<()> {
-    db.query("DROP FUNCTION IF EXISTS reshape.is_old_schema;")?;
+    db.query("DROP FUNCTION IF EXISTS reshape.is_new_schema;")?;
     Ok(())
 }

src/lib.rs

@@ -189,7 +189,8 @@ fn migrate(
 
     println!("Applying {} migrations\n", remaining_migrations.len());
 
-    helpers::set_up_helpers(db, &current_migration).context("failed to set up helpers")?;
+    let target_migration = remaining_migrations.last().unwrap().name.to_string();
+    helpers::set_up_helpers(db, &target_migration).context("failed to set up helpers")?;
 
     let mut new_schema = Schema::new();
     let mut last_migration_index = usize::MAX;
@@ -244,7 +245,6 @@ fn migrate(
     }
 
     // Create schema and views for migration
-    let target_migration = remaining_migrations.last().unwrap().name.to_string();
     create_schema_for_migration(db, &target_migration, &new_schema)
         .with_context(|| format!("failed to create schema for migration {}", target_migration))?;
 

src/migrations/add_column.rs

@@ -106,7 +106,7 @@ impl Action for AddColumn {
                 CREATE OR REPLACE FUNCTION {trigger_name}()
                 RETURNS TRIGGER AS $$
                 BEGIN
-                    IF reshape.is_old_schema() THEN
+                    IF NOT reshape.is_new_schema() THEN
                         DECLARE
                             {declarations}
                         BEGIN

src/migrations/alter_column.rs

@@ -103,7 +103,7 @@ impl Action for AlterColumn {
                 CREATE OR REPLACE FUNCTION {up_trigger}()
                 RETURNS TRIGGER AS $$
                 BEGIN
-                    IF reshape.is_old_schema() THEN
+                    IF NOT reshape.is_new_schema() THEN
                         DECLARE
                             {declarations}
                             {existing_column} public.{table}.{existing_column_real}%TYPE := NEW.{existing_column_real};
@@ -121,7 +121,7 @@ impl Action for AlterColumn {
                 CREATE OR REPLACE FUNCTION {down_trigger}()
                 RETURNS TRIGGER AS $$
                 BEGIN
-                    IF NOT reshape.is_old_schema() THEN
+                    IF reshape.is_new_schema() THEN
                         DECLARE
                             {declarations}
                             {existing_column} public.{table}.{temp_column}%TYPE := NEW.{temp_column};

src/migrations/common.rs

@@ -72,8 +72,6 @@ impl ToSql for PostgresRawValue {
 pub fn batch_touch_rows(db: &mut dyn Conn, table: &str, column: &str) -> anyhow::Result<()> {
     const BATCH_SIZE: u16 = 1000;
 
-    db.query("SET reshape.is_old_schema = 'YES'")?;
-
     let mut cursor: Option<PostgresRawValue> = None;
 
     loop {
@@ -152,8 +150,6 @@ pub fn batch_touch_rows(db: &mut dyn Conn, table: &str, column: &str) -> anyhow:
         cursor = last_value
     }
 
-    db.query("SET reshape.is_old_schema = ''")?;
-
     Ok(())
 }
 

src/migrations/remove_column.rs

@@ -61,7 +61,7 @@ impl Action for RemoveColumn {
                 CREATE OR REPLACE FUNCTION {trigger_name}()
                 RETURNS TRIGGER AS $$
                 BEGIN
-                    IF NOT reshape.is_old_schema() IS NULL THEN
+                    IF reshape.is_new_schema() THEN
                         DECLARE
                             {declarations}
                         BEGIN