Custom validation matcher

Created 10/15/2007, updated 3/12/20242 minute read

Thanks to FixtureReplacement my Rails model specs are looking awfully clean:

describe User, 'validating the login name' do
  it 'should require it to be present' do
     u = new_user(:login_name => nil)
     u.should_not be_valid
     u.should have_at_least(1).errors_on(:login_name)
  end

  # examples continue here...

end

Note how the new_user method provided by FixtureReplacement allows me to focus only on the salient attributes for the example; in this case that’s the overridden login_name attribute. Constructing new model instances is a breeze in this way, and the spec itself is much more intention-revealing because the salient bits and nothing else are right there up front.

And I’ve just whipped together a custom validation matcher to make it this kind of spec even clearer:

describe User, 'validating the login name' do
  it 'should require it to be present' do
     u = new_user(:login_name => nil)
     u.should fail_validation_for(:login_name)
  end

  # another example, this one written on a single line
  it 'should disallow trailing spaces' do
    new_user(:login_name => 'foobar ').should fail_validation_for(:login_name)
  end

  # examples continue here...

end

Here’s the custom matcher, which I’ve stuck in my spec/spec_helper.rb file:

module Spec
  module Rails
    module Matchers
      class FailValidationFor # :nodoc:
        def initialize attribute
          @attribute = attribute
        end

        def matches? model
          @model = model
          !@model.valid? and !@model.errors.on(@attribute).nil?
        end

        def failure_message
          "expected to fail validation with errors on #{@attribute} but was #{self.valid}; #{self.errors}"
        end

        def negative_failure_message
          "expected to pass validation with no errors on #{@attribute} but was #{self.valid}; #{self.errors}"
        end

        def description
          "fail validation for attribute #{@attribute}"
        end

        def valid
          if @model.valid?
            'valid'
          else
            'invalid'
          end
        end

        def errors
          if @model.valid?
            'no errors'
          else
            @model.errors.full_messages.to_sentence
          end
        end
      end # class FailValidationFor

      def fail_validation_for attribute
        FailValidationFor.new attribute
      end
    end # module Matchers
  end # module Rails
end # module Spec

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