Removing an existing deferrable constraint, and fail test if any constraint is deferrable. (#5806)

* Removing an existing deferrable constraint.

- Also adding a check to make sure the final dump contains no DEFERs.

* Spelling error

crates/db_schema_file/src/diff_check.rs

@@ -13,7 +13,7 @@ use std::{
 
 /// Returns almost all things currently in the database, represented as SQL statements that would
 /// recreate them.
-pub fn get_dump() -> String {
+pub(crate) fn get_dump() -> String {
   let db_url = SETTINGS.get_database_url();
   let output = Command::new("pg_dump")
     .args([
@@ -58,7 +58,7 @@ pub fn get_dump() -> String {
 /// not `dumps[1]` to `dumps[0]`. This requires the two `dumps` elements being in an order that fits
 /// with `label_of_change_from_0_to_1`. This does not necessarily match the order in which the dumps
 /// were created.
-pub fn check_dump_diff(dumps: [&str; 2], label_of_change_from_0_to_1: &str) {
+pub(crate) fn check_dump_diff(dumps: [&str; 2], label_of_change_from_0_to_1: &str) {
   let [sorted_statements_in_0, sorted_statements_in_1] = dumps.map(|dump| {
     dump
       .split("\n\n")
@@ -195,6 +195,13 @@ fn display_change([before, after]: [&str; 2]) -> impl Iterator<Item = &str> {
     })
 }
 
+/// Makes sure the after dump does not contain any DEFERRABLE constraints.
+pub(crate) fn deferr_constraint_check(dump: &str) {
+  if dump.contains(" DEFERR") {
+    panic!("Schema should not have DEFER constraints.")
+  }
+}
+
 // `#[cfg(test)]` would be redundant here
 mod tests {
   #[test]

crates/db_schema_file/src/schema_setup.rs

@@ -234,6 +234,8 @@ pub fn run(options: Options) -> LemmyResult<Branch> {
       let after = diff_check::get_dump();
 
       diff_check::check_dump_diff([&before, &after], "The code in crates/db_schema/replaceable_schema incorrectly created or modified things outside of the `r` schema, causing these changes to be left behind after dropping the schema:");
+
+      diff_check::deferr_constraint_check(&after);
     }
 
     run_replaceable_schema(&mut conn)?;

migrations/2025-06-18-130813_community_instance_not_deferrable/down.sql

@@ -0,0 +1,3 @@
+ALTER TABLE community
+    ALTER CONSTRAINT community_instance_id_fkey DEFERRABLE INITIALLY DEFERRED;
+

migrations/2025-06-18-130813_community_instance_not_deferrable/up.sql

@@ -0,0 +1,6 @@
+-- We should remove existing deferrable constraints, as they're potentially dangerous.
+--
+-- This is the only one I could find after doing a DB dump.
+ALTER TABLE community
+    ALTER CONSTRAINT community_instance_id_fkey NOT DEFERRABLE;
+