# All the places I used !! in a very sizable project
# (4 cases in 7600 LOC, about 40% of which is specs)

# ActiveRecord callback - writing to a DB:

  def update_has_emailable_accounts!
    # !! replaces "condition ? true : false"
    self.update_attribute(:has_emailable_accounts, !!accounts.detect { |account| account.emailable? })
  end

# AR validation helper predicate:

  def valid_url?(url)
    !! (url =~ %r[^https?://.]i)
  end

# application.rb - user? predicate:

  def user?
    !! current_user
  end
  helper_method :user?

# In a custom validation I wrote - returns true/false instead of... true/nil.
# I'd only justify this because its a predicate, but this is certainly
# my least solid usage.

    def has_uniqueness_errors?
      !! @has_uniqueness_errors
    end