Scoped Uniqueness Validation

Let's say you have a blog, with Authors and BlogPosts. A BlogPost has an Author and a title. If we have a requirement that the post titles need to be unique, our model would look something like this:

class Blog < ApplicationRecord
  has_one :author

  validates :title, uniqueness: true
end

Let's say the requirement changes, and the post title needs to be unique to the author (yeah it's convoluted, but let's roll with it). So Ann can author a post titled 'Cool Post', and Bob can also author 'Cool Post', but Ann can't publish another post titled 'Cool Post'.

Conveniently, uniqueness validations can be scoped to a particular attributes. So to satisfy the above, we can update the validation to:

class Blog < ApplicationRecord
  has_one :author

  validates :title, uniqueness: {scope: :author_id}
end