Proposal: before_begin/after_commit callbacks for migrations

[Nassim ABDELGHANI]

Following up on this discussion https://github.com/elixir-ecto/ecto_sql/issues/589#issuecomment-1902088320

Some sqlite schema changes need a command to run before the migration transaction opens. The procedure in the sqlite docs first disables the foreign key check with PRAGMA foreign_keys https://sqlite.org/lang_altertable.html#making_other_kinds_of_table_schema_changes

It is a no-op inside a transaction, that's why it has to happen outside.

One solution currently is to set @disable_ddl_transaction true, running the PRAGMA, then opening a transaction by hand in the migration. However that way, the migration transaction commits inside up/0, and the schema_migrations row is only written afterwards, outside the transaction.

If something fails in between, the schema has changed but the migration isn't recorded. Running it again either fails with a confusing error ("there is already another table with this name") or applies the migration a second time.

Is there a way to run a migration inside a transaction together with the schema_migrations update, while running the required PRAGMA outside?

If there is not, the proposal would be optional before_begin/0 and after_commit/0 callbacks, run outside the transaction but on the same checked-out connection, so a PRAGMA set in before_begin applies to the transaction that follows.

What do you think?

[José Valim]

Yes, please do send a PR!

José Valim

https://dashbit.co/

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