Add canonical URL and hreflang overrides to SEO helpers

app/helpers/better_together/application_helper.rb

@@ -80,6 +80,12 @@ def host_community_logo_url
       rails_storage_proxy_url(attachment)
     end
 
+    # Returns the canonical URL for the current request, allowing overrides via
+    # +content_for(:canonical_url)+.
+    def canonical_url
+      content_for?(:canonical_url) ? content_for(:canonical_url) : request.original_url
+    end
+
     # Builds SEO-friendly meta tags for the current view. Defaults are derived
     # from translations and fall back to the Open Graph description when set.
     # rubocop:todo Metrics/MethodLength
@@ -94,9 +100,17 @@ def seo_meta_tags # rubocop:todo Metrics/AbcSize, Metrics/MethodLength
 
       keywords = content_for?(:meta_keywords) ? content_for(:meta_keywords) : nil
 
+      canonical = canonical_url
+      hreflang_tags = I18n.available_locales.map do |locale|
+        tag.link(rel: 'alternate', hreflang: locale, href: url_for(locale:, only_path: false))
+      end
+      hreflang_tags << content_for(:hreflang_links) if content_for?(:hreflang_links)
+
       tags = []
       tags << tag.meta(name: 'description', content: description)
       tags << tag.meta(name: 'keywords', content: keywords) if keywords.present?
+      tags << tag.link(rel: 'canonical', href: canonical)
+      tags.concat(hreflang_tags)
 
       safe_join(tags, "\n")
     end
@@ -121,7 +135,7 @@ def open_graph_meta_tags # rubocop:todo Metrics/AbcSize, Metrics/MethodLength, M
                          t('og.default_description', platform_name: host_platform.name)
                        end
 
-      og_url = content_for?(:og_url) ? content_for(:og_url) : request.original_url
+      og_url = content_for?(:og_url) ? content_for(:og_url) : canonical_url
 
       og_image = content_for?(:og_image) ? content_for(:og_image) : host_community_logo_url
 

docs/seo.md

@@ -0,0 +1,27 @@
+# SEO
+
+The engine exposes helpers that output common search engine optimisation tags.
+
+## Canonical URL
+
+`seo_meta_tags` includes a canonical `<link>` tag pointing to the current request
+URL. You can override the URL by setting a `content_for` block:
+
+```erb
+<% content_for :canonical_url, article_url(@article, locale: :en) %>
+```
+
+## Hreflang links
+
+Alternate language links are generated for each available locale. Additional
+links can be appended using `content_for :hreflang_links` and are merged with the
+default set:
+
+```erb
+<% content_for :hreflang_links do %>
+  <%= tag.link rel: 'alternate', hreflang: 'x-default', href: root_url %>
+<% end %>
+```
+
+Links supplied via `content_for` are merged with the automatically generated
+links rather than replacing them.

spec/helpers/better_together/application_helper_spec.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+module BetterTogether
+  RSpec.describe ApplicationHelper, type: :helper do
+    before do
+      allow(helper).to receive(:host_platform).and_return(double(name: 'Test Platform', cache_key_with_version: 'test-platform'))
+      allow(helper).to receive(:host_community_logo_url).and_return(nil)
+      allow(controller.request).to receive(:original_url).and_return('http://test.host/en/current')
+      allow(I18n).to receive(:available_locales).and_return(%i[en fr])
+      allow(helper).to receive(:url_for) do |opts|
+        "http://test.host/#{opts[:locale]}/current"
+      end
+    end
+
+    describe '#seo_meta_tags' do
+      it 'includes default canonical and hreflang links' do
+        html = helper.seo_meta_tags
+        expect(html).to include('<link rel="canonical" href="http://test.host/en/current"')
+        expect(html).to include('<link rel="alternate" hreflang="fr" href="http://test.host/fr/current"')
+      end
+
+      it 'merges content_for overrides' do
+        helper.content_for(:canonical_url, 'http://example.com/custom')
+        helper.content_for(:hreflang_links, tag.link(rel: 'alternate', hreflang: 'es', href: 'http://test.host/es/current'))
+
+        html = helper.seo_meta_tags
+        expect(html).to include('<link rel="canonical" href="http://example.com/custom"')
+        expect(html).to include('<link rel="alternate" hreflang="fr" href="http://test.host/fr/current"')
+        expect(html).to include('<link rel="alternate" hreflang="es" href="http://test.host/es/current"')
+      end
+    end
+
+    describe '#open_graph_meta_tags' do
+      it 'defaults og:url to canonical_url' do
+        allow(helper).to receive(:canonical_url).and_return('http://test.host/en/current')
+        html = helper.open_graph_meta_tags
+        expect(html).to include('<meta property="og:url" content="http://test.host/en/current"')
+      end
+
+      it 'allows og_url override' do
+        helper.content_for(:og_url, 'http://example.com/og')
+        allow(helper).to receive(:canonical_url).and_return('http://test.host/en/current')
+        html = helper.open_graph_meta_tags
+        expect(html).to include('<meta property="og:url" content="http://example.com/og"')
+      end
+    end
+  end
+end