On Mon, Feb 22, 2010 at 4:54 PM, Grary <grary.sti...@gmail.com> wrote: > Thanks Fred. > > What do you mean by 'use a unique index'? > > What I am concerned about is logical uniqueness, i.e., persist only > data objects with a unique combination of attributes. >
So if I understand correctly, lets say there are two attributes a, and b You want to make sure that no two models have the same COMBINATION of values for a and b, so having two models with: a = 1, b = 2 a = 1, b = 3 would not be a conflict If that's the case then the standard validation class Widget < ActiveRecord::Base validates_uniqueness_of :a, :b end wouldn't work since it tries to prevent saving two models with the same value of a, OR with the same value of b And even if that's not what you're trying to do, and you're ok with the example being a conflict, Fred's point is that validates_uniqueness_of doesn't guarantee uniqueness if two users try to save conflicting records simultaneously. The validation works by first trying to find a record with the value, and if it doesn't find it inserting the 'new' record, and this can fail due to a concurrency hole. To fill this hole requires leaning on the database server, and the way to do that in SQL is by having a unique index on the table which covers the column or columns you want to be unique. This assume you are using a database which supports it, e.g. MySql. To create an index you can create a migration which includes a statement like add_index :widgets, [:a, :b], :unique => true) Assuming that the table name for the model is 'widgets' Now if you do this, you also need to be aware that if you try to save a record with a uniqueness conflict the save will raise an ActiveRecord::StatementInvalid exception, which you'll need to rescue and do something like telling the user of the conflict so that he can resolve it. -- Rick DeNatale Blog: http://talklikeaduck.denhaven2.com/ Twitter: http://twitter.com/RickDeNatale WWR: http://www.workingwithrails.com/person/9021-rick-denatale LinkedIn: http://www.linkedin.com/in/rickdenatale -- You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to rubyonrails-t...@googlegroups.com. To unsubscribe from this group, send email to rubyonrails-talk+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en.
