[FIXED] Help me clean this up so one scope uses the other!

article.rb

# FIXED: This is what I did in the end, after discussing it with @dcadenas.
# Check the previous versions of the gist to see how the problem looked before :)
#
# I have this AR model that has two scopes that should return complementary sets
# of rows: one returns all the rows that match certain conditions, the other
# returns all rows that DON'T match the same conditions.
#
# I would like to build the second scope using the first one. Something like:
#
#     class Article
#       def self.problematic
#         not(complete)
#       end
#     end
#
# For now the conditions are simple enough (just check for NULL/NOT NULL), but
# it's probable that I might need some more complex conditions in the future
# (like published_on < Time.current, since our scraper sometimes messes up).
#
# So this scope will become more complex, and I would like to avoid having to
# duplicate all the logic between both scopes.
#
# After poking around ARel's source code for a while I didn't find an easy way
# to do this. How would you approach this?

class Article < ActiveRecord::Base
  # Public: Filter out articles that don't have all the information required for
  # us to show them in the store/reader.
  #
  # Returns an ActiveRecord::Relation.
  def self.complete
    where(_conditions_for_complete)
  end

  # Public: Filter articles that don't have all the information required for us
  # to show them in the store. This is the opposite of Article.complete.
  #
  # Returns an ActiveRecord::Relation.
  def self.problematic
    where(_conditions_for_complete.not)
  end

  # ARel conditions to filter articles that are complete or not. This can be
  # later negated to compose both .complete and .problematic scopes.
  #
  # Returns an Arel::Node.
  def self._conditions_for_complete
    conditions = [:published_on, :title, :text, :url].map do |field|
      arel_table[field].not_eq(nil)
    end
    conditions.inject(:and)
  end
end