Rails 3 app model how ensure only one boolean field set to true at a time

Question

I have a Logo model that has fields of name:string, default:boolean. I want the true value to be unique, so that only one item in the database can be set to true at once. How do

Answer 1

If you're coming here in a more recent time and are using Rails 6, this should be covered on the database level as well as the model level:

db level:

add_index :items, :default, unique: true, where: '(default IS TRUE)', algorithm: :concurrently

model level:

class Item < ApplicationRecord
  scope :default, -> { where(default: true) }

  validates :default, uniqueness: { conditions: -> { default } }
end