An RSpec matcher for validating Rails <meta/> tags

While writing a project for Wilcox Tech that we hope to open source soon, I had reason to test the content (or, in some cases, presence) of <meta/> tags in a Rails app.

I came across a good start in dB.’s blog article, Custom RSpec Meta Tag Validator. However, the article was from 2013, did not follow the modern RSpec protocol, and the matcher felt a bit off for my purposes.

What I really wanted was a matcher that felt like the built-in ones to Rails, such as assert_dom or assert_select. So, using dB.’s XPath query as a starting point, I wrote a new one.

I am under the assumption that dB.’s snippet is licensed CC-BY as is the rest of his post. I am therefore licensing my matcher under a dual-license, as I believe this to be legal: CC-BY with my authorship, or MIT (the standard license of most Rails-related Ruby Gems). To fully comply, you will need to acknowledge him as well.

Usage looks like this:

  describe 'meta product properties' do
    before { assign(:item, create(:item, description: 'This is a test description.', price: 50.50)) }

    it 'has the correct Open Graph type' do
      render_item
      expect(rendered).to have_meta 'og:type', 'product'
    end

   it 'has the description set' do
      render_item
      expect(rendered).to have_meta 'og:description', 'This is a test description'
    end

    it 'has the price set' do
      render_item
      expect(rendered).to have_meta 'product:price.amount', '50.50'
    end
  end

  describe 'meta image properties' do
    context 'with no photos' do
      it 'does not have an og:image property' do
        assign(:item, create(:item))
        render_item
        expect(rendered).not_to have_meta 'og:image'
      end
    end

    context 'with a photo' do
      before do
        item = create(:item)
        photo = mock_item_photo_for item
        photo.description = ‘My Photo Description’
        assign(:item, item)
      end

      it 'has an og:image property' do
        render_item
        expect(rendered).to have_meta 'og:image'
      end

      it 'has the og:image:alt property set to the photo description' do
        render_item
        expect(rendered).to have_meta 'og:image:alt', 'My Photo Description'
      end
    end
  end

And here’s my spec/support/matchers/have_meta.rb:

# frozen_string_literal: true

class HaveMeta
  attr_accessor :expected, :actual, :key

  def initialize(*args)
    raise ArgumentError, 'Need at least one argument' if args.empty?

    @key = args.shift
    @expected = args.shift
  end

  def diffable?
    @expected.present?
  end

  def matches?(page)
    meta = page.html.at_xpath("//head/meta[@property='#{@key}' or @name='#{@key}']")
    return false if meta.blank?

    return true if @expected.blank?

    @actual = meta['content']
    @actual.include? @expected
  end

  def failure_message
    return "expected meta property '#{key}'" if @expected.blank?

    "expected '#{key}' to contain '#{@expected}' in '#{@actual}'"
  end

  def failure_message_when_negated
    return "expected not to find meta property '#{key}'" if @expected.blank?

    "expected '#{key}' to not contain '#{@expected}' in '#{@actual}'"
  end
end

# Determines if the rendered page has a given meta tag.
def have_meta(*args)
  HaveMeta.new(*args)
end