Exchanging values of two records with unique-validated fields

rubyonrails-talk
1

Hi fellows!

Is there a way to atomically exchange values of records which are validated using ‘validates_uniqueness_of’?

Some details follow.

Imagine I have a model:

class Item < ActiveRecord::Base

validates_uniqueness_of :weight

end

‘weight’ is a sorting weight.

I have an index.html view which has a list of “Item” records wrapped in jQueryUI’s sortable container.

My goal is to do some ajax request (POST i guess) whenever user changes items’ order. I.e. i plan to do end up with controller action that calls function like:

class Item

def self.exchange_weights(id1, id2)

item1 = Item.find(id1)

item2 = Item.find(id2)

weight1 = Item.find(id1).weight

weight2 = Item.find(id2).weight

item1.weight = some_temp_weight

item2.weight = weight1

item1.weight = weight2

end

end

Is this the right way to do this?

Or can I somehow just do a separate POST requests on these Item’s instances by triggering ItemsController#update for each of them and passing weight - but here validations would not allow me to do this unless I miss something - that’s why I desided to ask :slight_smile:

Thanks.

2

If you mean “atomic” on the level of saving to the database

(all or nothing), you would need a database transaction.

http://api.rubyonrails.org/classes/ActiveRecord/Transactions/ClassMethods.html

Second, to circumvent the problem of the uniqueness validation,

I see 2 solutions:

  1. what you propose above:

UNTESTED:

def self.exchange_weights(id1, id2)

item1 = Item.find(id1)

item2 = Item.find(id2)

weight1 = item1.weight

weight2 = item2.weight

Item.transaction do

item1.weight = some_temp_weight

item1.save!

item2.weight = weight1

item2.save!

item1.weight = weight2

item1.save!

end # you still need to catch the exception here

end

This could fail … e.g. if 2 parallel processed each write the same

some_temp_weight exactly at the same time …

  1. Alternative:

Make the validation dependent on an instance variable

UNTESTED

class Item

attr_accessor :no_weight_uniqueness_validation

validates :weight, :uniqueness => true, :unless => @no_weight_uniqueness_validation

end

def self.exchange_weights(id1, id2)

item1 = Item.find(id1)

item2 = Item.find(id2)

weight1 = item1.weight

weight2 = item2.weight

Item.transaction do

item2.weight = weight1

item2.no_weight_uniqueness_validation = true

item2.save!

item1.weight = weight2

item1.save!

end # you still need to catch the exception here

end

This will NOT work if you also have the uniqueness validation

in the database itself … which you should, since the

uniqueness validation in Rails is not automatically protected

against race conditions when multiple parallell processes

write to the same database.

HTH,

Peter

3

Oh, it’s clearer now. And, yes I have just saw in rails’ apidocs that I should do add_index on the DB level too - to prevent possible clashes. Didn’t know that until now, thanks.

I guess that I will start with the first solution then. And maybe will improve it to be the latter if find the need too.

Thanks, Peter!