ActiveRecord not behaving as I expect it - mysql

Question

Ruby 2.0
Windows 8.1
Rails 4.1
MySQL2 gem
To avoid an error, I am using the following code to check for an existing payment, prior to creating a new payment record:
payment = {"organization_id" => organization_id,
"date" => row[1],
"amount" => row[2],
"description" => row[3]}
slug = "#{organization_id.to_s}_#{row[1].to_s}_#{row[2].to_s}_#{row[3]})
organization_payment = OrganizationPayment.where(:slug => slug)[0]
if !organization_payment
new_organization_payment = OrganizationPayment.create(payment)
end
Every once in a while, I am getting the following error:
Mysql2::Error at /process_uploaded_payments_data
Duplicate entry 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxx' for key 'index_organization_payments_on_slug'
I also have the following in my model:
validates_uniqueness_of :slug
Is there any reason why the entry causing the duplicate error would not have been caught by the code above? Any ideas?
Solution
I am still not certain what caused the problem, but I learned the hard way that validating uniqueness does not really work, if you also have a before_save call in your model that creates the slug in question. The workaround is an exception handler:
begin
new_organization_payment = OrganizationPayment.create(payment)
rescue ActiveRecord::RecordNotUnique
next
end

Answer 1

I don't know if this is your problem but a possible cause of this could be race condition -- when your code is running in a process it can be interrupted right after the if condition before it creates the new record.
Putting a unique constraint on the column in the database is a fine way of dealing with this problem, though. You can catch the exception and deal with it that way. You also don't have to manually check for the duplicity, you can use active record validations; fetching the entire record just to check if it exists is not the best practice anyway. More info:
http://apidock.com/rails/ActiveRecord/Validations/ClassMethods/validates_uniqueness_of