Don't use transactions when working with unsafed records in associations

[benedikt]

Currently rails wraps every operation on association collections in a transaction. This results in unecessary BEGIN COMMIT statements sent to the database server when working with unsaved records. In our application these unecessary statements caused about 70% of the whole database time.

Example:

f = Firm.new
f.clients_of_firm << Client.new("name" => "New Client")
# Results in an empty transaction statement in the database:
# BEGIN
# COMMIT

The same issue applies not only to CollectionAssociation#concat but also to CollectionAssociation#replace and CollectionAssociation#delete_or_destroy. This pull request fixes the issue by only wrapping the calls when the transaction is actually needed.

[josevalim]

@jonleighton I thought we removed the code that wraps all operations in a transaction?

[benedikt]

Of course this would solve this, too... but is this really what you want? Especially when adding/replacing/removing several records at once the transaction is quite handy, isn't it?

[jonleighton]

@josevalim We remove it for e.g. foo.items.build, but it looks like there are further issues...

@benedikt thanks for reporting and providing a patch! I want to work on the code a bit before merging, but I aim to have it merged by Sunday night UTC+1

@tenderlove giving you the heads up about this - I think this fix ought to go in 3.1 final as it's a perf regression. If the current RC has no "omg" issues then maybe we can create another RC with just an absolutely minimal set of important changes cherry-picked (e.g. this) and this have a shorter RC cycle for that.

[josevalim]

tks @jonleighton

[benedikt]

@jonleighton If you want me to change anything in the patch (or anything related) just tell me. :)

[jonleighton]

This is merged now:

• fa8dfad
• de01a64
• 14e6bbb
• b09bbdb

Thanks for offering to make changes. I would definitely have told you what I wanted changed if it was obvious, but I had to actually get in there and play with the code before I could work out how I wanted to change it!

Thanks for contributing.