validates_one_of

Question

I have a table that contains a column for a boolean value. I want there to be only one row that can have that boolean value be true. Does rails support such a feature?

I'm looking for something like the following:

Table  
id | bool_value
1  | false
2  | false
3  | true
4  | false

Answer 1

You can do this, which would only validate the uniqueness if enabled was set to true and would ignore other cases (nil and false), because nil == false:

validates_uniqueness_of :enabled, :if => :enabled

That should cover all cases.

Hope this helps!

Answer 2

validates_uniqueness_of :enabled, :if => :enabled is a pretty hack, but you have a problem with your design if you are spreading one conceptual bit of true/falseness across multiple records. This is going to perform/scale poorly, and can allow for inconsistency to creep in.

Lets look at some alternatives in a world where only one Answer can be Top Answer.

Scenario 1: Instead of an is_top_answer column on Answer, you have in some other table a field called top_answer_id, referring to an answer.

class Site; has_one :top_answer, :class => :answer; end

Clearly now there can be only one, and setting and getting become trivially easy, compared to the table scans your design requires.

Scenario 2: Answer has a foreign key to Question, and there can only be one top answer per question. You can extend the previous solution by having top_answer_id in the question table. Its common that the previous scenario will grow into this one over time.

Depending on your needs, you may also want to consider not rejecting a second boolean being true, in an after_save (part of the same transaction), simply setting all others to false. It's hardly more work than scanning the entire table to see whether it's allowed to set the current record to true.