diesel_migrations fails when search_path is set in migration script

[kairoswater-jason]

Setup

Any migration script similar to the following
up.sql

SET search_path='foo';

--alternatively you could use:
--SELECT pg_catalog.set_config('search_path', '', false);

Versions

• Rust: 1.90
• Diesel:

diesel = { version = "2.2.0", features = ["postgres"] }
diesel_migrations = { version = "2.2.0", features = ["postgres"] }

• Database: postgres
• Operating System linux

Feature Flags

• diesel: postgres
• diesel_migrations: postgres

Problem Description

diesel_migrations fails when search_path is set in migration script

What are you trying to accomplish?

Perform a migration where the search_path is set

This allows using shortened names for things like types

What is the expected output?

$ diesel migration run
Running migration 2025-08-27-111711_initial
$

What is the actual output?

$ diesel migration run
Running migration 2025-08-27-111711_initial
2025-08-27T18:52:00.647148Z ERROR diesel::database: Failed to execute query query=Query { sql: "INSERT INTO \"__diesel_schema_migrations\" (\"version\") VALUES ($1)", binds: [] } err=DatabaseError(Unknown, "relation \"__diesel_schema_migrations\" does not exist")
Failed to run migrations: relation "__diesel_schema_migrations" does not exist
$

Are you seeing any additional errors?

n/a

Steps to reproduce

• create a migration script containing SET search_path='foo' or similar
• run migration script with diesel migration run or any other method (eg. programmatically is expected to perform similarly to CLI)

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.
• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)
• This issue can be reproduced without requiring a third party crate

[weiznich]

Thanks for reporting this issue. This seems to be a rather specific issue. As far as I can tell there is no easy way for diesel to detect that a migration changes the search path, therefore I would like to document that changing this setting is not allowed inside of a migration. We are happy to review a PR for this.

[kairoswater-jason]

Does it really need to detect anything?

I wonder if it might be possible for diesel to set the search path to whatever value it currently assumes it to be before it runs migrations?

[weiznich]

Given that at least this kind of search path setting is only a session wide setting that might work. We are happy to receive a PR including test for that.

[kairoswater-jason]

as a sanity check: do we know what value it currently assumes search_path to be?

(I'm hoping to make sure I know what to expect before possibly going off and doing the work/testing/etc. - I hesitate to assume anything about how it works having not dug into it yet)

[weiznich]

No, diesel doesn't record that at all. You would like need to change these functions:

diesel/diesel_migrations/src/migration_harness.rs

Lines 175 to 226 in b473a10

	fn run_migration(
	&mut self,
	migration: &dyn Migration<DB>,
	) -> Result<MigrationVersion<'static>> {
	let apply_migration = |conn: &mut C| -> Result<()> {
	migration.run(conn)?;
	diesel::insert_into(__diesel_schema_migrations::table)
	.values(
	__diesel_schema_migrations::version.eq(migration.name().version().as_owned()),
	)
	.execute(conn)?;
	Ok(())
	};
	if migration.metadata().run_in_transaction() {
	self.transaction(apply_migration)?;
	} else {
	apply_migration(self)?;
	}
	Ok(migration.name().version().as_owned())
	}
	fn revert_migration(
	&mut self,
	migration: &dyn Migration<DB>,
	) -> Result<MigrationVersion<'static>> {
	let revert_migration = |conn: &mut C| -> Result<()> {
	migration.revert(conn)?;
	diesel::delete(
	__diesel_schema_migrations::table.find(migration.name().version().as_owned()),
	)
	.execute(conn)?;
	Ok(())
	};
	if migration.metadata().run_in_transaction() {
	self.transaction(revert_migration)?;
	} else {
	revert_migration(self)?;
	}
	Ok(migration.name().version().as_owned())
	}
	fn applied_migrations(&mut self) -> Result<Vec<MigrationVersion<'static>>> {
	setup_database(self)?;
	Ok(__diesel_schema_migrations::table
	.into_boxed()
	.select(__diesel_schema_migrations::version)
	.order(__diesel_schema_migrations::version.desc())
	.load(self)?)
	}
	}

To:

• Check if the backend is Pg, eg. via TypeId
• Record the search path at the start of each migration via SHOW search_path; via diesel::sql_query or similar
• At the end of each migration reset the search path via SET search_path = $1 and diesel::sql_query to the recorded value.

For testing the easiest solution is to just add a print-schema test here that sets the search path.