Ruby on Rails

You need to enforce the uniqueness at the database level, then catch errors.

https://robots.thoughtbot.com/the-perils-of-uniqueness-validations

More info on transactions here:

https://makandracards.com/makandra/31937-differences-between-transactions-and-locking

iiuc - transaction enforces that all changes within the transaction are applied (or none). 

it doesn't enforce that changes in transaction B are applied before a lookup in transaction A happens

looks like you could also work with a lock, though the database enforcement seems more natural.

On Tuesday, 9 October 2018 22:03:35 UTC+1, belgoros wrote:

I can't figure out the right way to use find_or_create_by method which is not atomic.

In short, I have a before_action filter is used to either to find or create a User by its username. 

def user

    if decoded_auth_token && decoded_auth_token[:sub]

      @user ||= User.find_or_create_by!(username: decoded_auth_token[:sub])

      Rails.logger.silence do

        @user.update_column(:token, http_auth_header)

      end

      @user

    end

  rescue ActiveRecord::RecordInvalid => e

    raise(

      ExceptionHandler::InvalidToken,

      ("#{Message.invalid_token} #{e.message}")

    )

  end

The problem is that the above method is called twice by different threads: 2 requests com from a JS front-end app, and I have the situation when 2 Users are created with the same username.

I tried to apply the suggested solution and wrap the method call in a transaction and use retry:

begin    user = User.transaction(requires_new: true) do      User.find_or_create_by(username: some_value)    end  rescue ActiveRecord::RecordNotUnique    retry  end

call_some_method to update other user attributes in User model:

def update_user_info(options)      identifier = normalize_identifier(options[:sitenumber])      update(        first_name: options[:givenName],        last_name: options[:sn],        shop_identifier: identifier,        shop: user_shop(identifier)      )    end

but it creates nevertheless the duplicate record. What am I missing ? 

Used Rails version: 5.2.0

Ruby: 2.5.0

Thank you.