Ruby & Datamapper checking if a record exists, and where?

I have a basic Ruby app that I am building with Sinatra, Datamapper and has user authentication using OAuth. When I receive the data back from the Oauth service, I save a record of a new user in an sqlite3 db.

What I don't know how to do is how to go about verifying the user record doesn't already exist on the user database table. I can use the user's unique id (uid) to cross check whether the uid is already stored, but I am just unsure where to do this.

I have 2 classes and a /callback route. The User class is the db model, and an Authentication class has assorted methods for connecting to the OAuth, and the /callback route which will have the Authentication.save method being called.

Should I be checking for an existing record within the Authentication.save method and return a boolean or something else? Create a new method in Authentication that would be like Authentication.exists? (and what would that look like?) Or should I be checking within the /callback route?

I apologize if this wasn't 100% clear, I am having a difficult time describing my issue and am an absolute Ruby开发者_开发问答 beginner...

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Just before creating your new user, you should try to check if an user with the same login and email (for example) exists :

require 'dm-aggregates'

user=User.new
user.login=your_login_data_returned_by_oauth
user.email=your_email_data_returned_by_oauth
# And so on...
user.save if User.count(:login=>user.login, :email=>user.email) == 0