```ruby
module Shoulda # :nodoc:
  module Matchers
    module ActiveRecord # :nodoc:

      # Ensures that the attribute cannot be changed once the record has been
      # created.
      #
      #   it { should have_readonly_attributes(:password) }
      #
      def have_readonly_attribute(value)
        HaveReadonlyAttributeMatcher.new(value)
      end

      class HaveReadonlyAttributeMatcher # :nodoc:

        def initialize(attribute)
          @attribute = attribute.to_s
        end

        def matches?(subject)
          @subject = subject
          if readonly_attributes.include?(@attribute)
            @negative_failure_message =
              "Did not expect #{@attribute} to be read-only"
            true
          else
            if readonly_attributes.empty?
              @failure_message = "#{class_name} attribute #{@attribute} " <<
                "is not read-only"
            else
              @failure_message = "#{class_name} is making " <<
                "#{readonly_attributes.to_a.to_sentence} " <<
                "read-only, but not #{@attribute}."
            end
            false
          end
        end

        attr_reader :failure_message, :negative_failure_message

        def description
          "make #{@attribute} read-only"
        end

        private

        def readonly_attributes
          @readonly_attributes ||= (@subject.class.readonly_attributes || [])
        end

        def class_name
          @subject.class.name
        end

      end

    end
  end
end
```

Fix documentation typo. Method is named have_readonly_attribute, but documentation refers to have_readonly_attributes.
user
```ruby
module Shoulda # :nodoc:
  module Matchers
    module ActiveRecord # :nodoc:

      # Ensures that the attribute cannot be changed once the record has been
      # created.
      #
      #   it { should have_readonly_attribute(:password) }
      #
      def have_readonly_attribute(value)
        HaveReadonlyAttributeMatcher.new(value)
      end

      class HaveReadonlyAttributeMatcher # :nodoc:

        def initialize(attribute)
          @attribute = attribute.to_s
        end

        def matches?(subject)
          @subject = subject
          if readonly_attributes.include?(@attribute)
            @negative_failure_message =
              "Did not expect #{@attribute} to be read-only"
            true
          else
            if readonly_attributes.empty?
              @failure_message = "#{class_name} attribute #{@attribute} " <<
                "is not read-only"
            else
              @failure_message = "#{class_name} is making " <<
                "#{readonly_attributes.to_a.to_sentence} " <<
                "read-only, but not #{@attribute}."
            end
            false
          end
        end

        attr_reader :failure_message, :negative_failure_message

        def description
          "make #{@attribute} read-only"
        end

        private

        def readonly_attributes
          @readonly_attributes ||= (@subject.class.readonly_attributes || [])
        end

        def class_name
          @subject.class.name
        end

      end

    end
  end
end
```
assistant