Rick, Thanks. That sounds exactly like what I'm after...
Grary On Feb 22, 8:30 pm, Rick DeNatale <[email protected]> wrote: > On Mon, Feb 22, 2010 at 4:54 PM,Grary<[email protected]> 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 [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en.
