Add a canonical URL meta tag

This is an optional meta tag which defaults to not being shown.
It's built from the base_path of the content item which means it
strips out any query parameters or fragments.

CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+
+* Add an optional `canonical` meta tag.
+
 # 7.2.0
 
 * Add department colours to components (PR #296)

app/views/govuk_publishing_components/components/docs/meta_tags.yml

@@ -23,3 +23,13 @@ examples:
         links:
           world_locations:
           - analytics_identifier: WL3
+  with_default_canonical_path:
+    data:
+      content_item:
+        base_path: /test
+      canonical_path: true
+  with_overridden_canonical_path:
+    data:
+      content_item:
+        base_path: /test
+      canonical_path: /this-is-a-test-path

lib/govuk_publishing_components/presenters/meta_tags.rb

@@ -20,6 +20,7 @@ def meta_tags
         meta_tags = add_political_tags(meta_tags)
         meta_tags = add_taxonomy_tags(meta_tags)
         meta_tags = add_step_by_step_tags(meta_tags)
+        meta_tags = add_canonical_tag(meta_tags)
         meta_tags
       end
 
@@ -106,6 +107,20 @@ def add_step_by_step_tags(meta_tags)
         meta_tags
       end
 
+      def add_canonical_tag(meta_tags)
+        if local_assigns.key?(:canonical_path)
+          canonical_path = if local_assigns[:canonical_path] == true
+                             content_item[:base_path]
+                           else
+                             local_assigns[:canonical_path]
+                           end
+
+          meta_tags["canonical"] = Plek.new.website_root + canonical_path
+        end
+
+        meta_tags
+      end
+
       def has_content_history?
         (content_item[:public_updated_at] && details[:first_public_at] && content_item[:public_updated_at] != details[:first_public_at]) ||
           (details[:change_history] && details[:change_history].size > 1)

spec/components/meta_tags_spec.rb

@@ -31,6 +31,30 @@ def example_document_for(schema_name, example_name)
     assert_meta_tag('govuk:schema-name', 'publication')
   end
 
+  it "renders the canonical URL in a meta tag if requested" do
+    render_component(
+      content_item: { base_path: "/test" },
+      canonical_path: true,
+    )
+
+    assert_meta_tag('canonical', 'http://www.dev.gov.uk/test')
+  end
+
+  it "renders the canonical URL in a meta tag if overridden" do
+    render_component(
+      content_item: { base_path: "/test" },
+      canonical_path: "/test2",
+    )
+
+    assert_meta_tag('canonical', 'http://www.dev.gov.uk/test2')
+  end
+
+  it "doesn't renders the canonical URL in a meta tag if not requested" do
+    assert_empty render_component(content_item: {
+      base_path: "/test"
+    }).strip
+  end
+
   it "renders organisations in a meta tag with angle brackets" do
     content_item = {
       links: {