Transactions can be turned off per Migration

[senny]

Closes #9483.

There are SQL Queries that can't run inside a transaction. Since
the Migrator used to wrap all Migrations inside a transaction there
was no way to run these queries within a migration.

This patch adds self.with_transaction = false to the migration to
turn transactions off when necessary.

[senny]

@rafaelfranca @carlosantoniodasilva what do you think?

[carlosantoniodasilva]

... inside a transaction though, and for... wdyt?

[senny]

👍

[carlosantoniodasilva]

I'm not sure if I like with_transaction... How about: use_transaction = false, or skip_transaction = true, or even skip_ddl_transaction.

[senny]

From the wording I would go with skip_transaction but I don't like the fact that it's a negative option I would like something that has the meaning of:

• true => with transaction.
• false => without transactions.

[carlosantoniodasilva]

Does it need to be an option?

[senny]

My first implementation did not use an option but a method call like without_transaction but in the end I liked the option better because in my opinion it's easier to understand what happens.

If you don't like with_transaction I would rename it to use_transaction=false but I don't have a very strong opinion about it. If you'd like to change it let me know.

[carlosantoniodasilva]

There's one extra space at the beginning of these lines. Also Migration can be downcased I think.

[carlosantoniodasilva]

Seems good @senny, I'm just not sure about the naming yet. with_transaction seems the best option so far, I though about wrap_in_transaction but that doesn't look that good. Wdyt, are we good with with_transaction?

[exviva]

Typo Remeber => Remember

[senny]

@rafaelfranca @carlosantoniodasilva I pushed an updated and rebased version:

• We now use a macro called disable_ddl_transaction!
• I added documentation to the migrations guide
• I edited the copy according to the feedback comments

[rafaelfranca]

Why this one?

[senny]

it's just a glitch in the diff, you see, it was removed on the top: https://github.com/rails/rails/pull/9507/files#L1L368

[rafaelfranca]

Ahhh. Right.

[carlosantoniodasilva]

👍

[Crisfole]

👍 Thanks Senny

[PhilCoggins]

@senny Our team came across this when trying to reduce memory consumption on a data migration for a larger table. It helped us out a lot, thank you for adding this!

We were curious why the name disable_ddl_transaction! was used, since this disables all transactions inside a migration, not just for DDL SQL statements. Perhaps you, or someone else could provide some insight on why you didn't use a more generic name like disable_transaction!.

[senny]

@PhilCoggins this name was chosen after some brainstorming. It expressed best what most people relate with the wrapping transaction. While it does not disable "all transactions", the wrapping transaction can contain non-ddl statements. You can still have transactions inside your migration when you use disable_ddl_transaction!. disable_transaction! might be to general, one name that came up during the initial discussion was self.transactional = false. We opted for disable_ddl_transaction! to communicate the meaning that that wrapping transaction is only in place for adapters that support ddl-transactions. For example it's not there with MySQL. Hope that makes sense.