loco-rs/loco · error

implement `up()` for the

Error message

implement `up()` for the `{{name}}` migration

What it means

A panic raised by a `todo!()` in a freshly generated Loco migration. When the generator cannot infer a schema change from the migration name (e.g. `cargo loco generate migration AddPostsTable` is too ambiguous), it emits an empty migration whose `up()` panics with this message so that a no-op migration is never silently recorded in `_loco_migrations`.

Solutions

1. Open migration/src/m<timestamp>_<name>.rs and implement `up()` using `loco_rs::schema` helpers (create_table, add_column, remove_column, rename_column, add_reference)
2. Give `down()` the inverse operation so rollback works
3. Alternatively delete the unused generated migration file and regenerate with a conventional name (e.g. CreatePosts) that the generator can infer
4. Re-run `cargo loco db migrate`

Example fix

// before
async fn up(&self, _m: &SchemaManager) -> Result<(), DbErr> {
    todo!("implement `up()` for the `AddPosts` migration")
}
// after
async fn up(&self, m: &SchemaManager) -> Result<(), DbErr> {
    m.create_table(
        Table::create()
            .table(Posts::Table)
            .col(ColumnDef::new(Posts::Id).integer().primary_key())
            .col(ColumnDef::new(Posts::Title).string().not_null())
            .to_owned(),
    ).await
}

grep -rL "todo!" migration/src/ || echo "some migrations are still stubs"

When it happens

Trigger: Running `cargo loco db migrate` (or anything that boots migrations) after generating a migration whose name the generator could not map to a schema change, without editing the `up()` body.

Common situations: Developers run `cargo loco generate migration` with a name that doesn't match Loco's naming conventions (e.g. `posts` instead of `CreatePosts`), then immediately run `db migrate`; CI pipelines fail on a migration stubbed earlier and forgotten.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• db cleanup thread panicked
• Type mismatch in RefGuard
• only enum supported
• failed to install Ctrl+C handler
• failed to install signal handler

AI-assisted analysis of loco-rs/loco@23639d1e36 (2026-09-12). Data as JSON: /api/errors/76ccdfb44bed57d0. Report an issue: GitHub.

Appendix: source

Thrown at loco-gen/src/templates/migration/empty.t:29

- into: "migration/src/lib.rs"
  before: "pub struct Migrator"
  content: "mod {{module_name}};"
---
use sea_orm_migration::{prelude::*, schema::*};

#[derive(DeriveMigrationName)]
pub struct Migration;

#[async_trait::async_trait]
impl MigrationTrait for Migration {
    async fn up(&self, m: &SchemaManager) -> Result<(), DbErr> {
        // Loco could not infer a schema change from the name `{{name}}`, so this
        // migration does nothing yet. Write the change here — the helpers in
        // `loco_rs::schema` (`add_column`, `remove_column`, `rename_column`,
        // `add_reference`, `create_table`, ...) cover the common cases — and
        // give `down()` the inverse. Until then `db migrate` panics here rather
        // than recording a migration that changed nothing.
        todo!("implement `up()` for the `{{name}}` migration")
    }

    async fn down(&self, m: &SchemaManager) -> Result<(), DbErr> {
        Ok(())
    }
}

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