Do not validate slug for drafts of existing published editions with sequenced slugs

If there are already published editions with slugs ending in `--2` and the like, we want to continue publishing at that slug.
Raising an error means users are basically unable to create new drafts without renaming at the moment.

There are around 12k documents with sequenced slugs, and fixing them manually may no be desirable to users who potentially depend on those slugs not changing for whatever reasons.
Cases in the wild are such that regenerating the slug "correctly" based on titles might end up redirecting to unexpected documents, as we cannot account for all the possible permutations of human error in the data.

```
whitehall(prod)> Document.where("slug LIKE ?", "%--%").count
=> 12246
```

Author: lauraghiorghisor-tw

lib/whitehall/publishing_api.rb

@@ -69,7 +69,7 @@ def self.save_draft_translation(
 
         content.merge!(bulk_publishing: true) if bulk_publishing
 
-        ensure_base_path_is_associated_with_this_content_id!(model_instance, content)
+        ensure_base_path_is_associated_with_this_content_id!(presenter.content_id, content[:base_path], model_instance)
 
         Services.publishing_api.put_content(presenter.content_id, content)
       end
@@ -181,22 +181,23 @@ def self.assert_public_edition!(instance)
       end
     end
 
-    def self.ensure_base_path_is_associated_with_this_content_id!(instance, content)
-      content_id = instance.content_id
+    def self.ensure_base_path_is_associated_with_this_content_id!(content_id, base_path, instance)
+      # Not all models that use this module respond to `base_path_without_sequence`.
+      base_path_without_sequence = instance.try(:base_path_without_sequence)
 
-      base_path = if instance.respond_to?(:base_path_without_sequence)
-                    # `base_path_without_sequence` required because if an edition with the same
-                    # title exists in Whitehall then `base_path` of `instance` will have
-                    # a unique sequence identifier and the check will not work as intended
-                    instance.base_path_without_sequence
-                  else
-                    content[:base_path]
-                  end
+      # If the edition is a first draft, then we should check that no existing content uses the base_path_without_sequence.
+      # Drafts with already taken slugs will have sequences appended to their base_paths, like '--2'.
+      # To provide the correct error message, we need to find the existing content at the base_path_without_sequence.
+      existing_content_id = if instance.published_major_version.nil?
+                              Services.publishing_api.lookup_content_id(base_path: base_path_without_sequence || base_path)
+                            else
+                              # If already published, then check only the published base_path (may or may not have sequence).
+                              Services.publishing_api.lookup_content_id(base_path: base_path)
+                            end
 
-      existing_content_id = Services.publishing_api.lookup_content_id(base_path:)
       return if existing_content_id.nil? || existing_content_id == content_id
 
-      raise UnpublishableInstanceError, "Cannot save draft (content_id #{content_id}). There is existing content at the '#{base_path}' route, under a different content_id (#{existing_content_id}). Try changing your title to resolve the conflict."
+      raise UnpublishableInstanceError, "Cannot save draft (content_id #{content_id}). There is existing content at the '#{base_path_without_sequence || base_path}' route, under a different content_id (#{existing_content_id}). Try changing your title to resolve the conflict."
     end
   end
 end

test/unit/lib/whitehall/publishing_api_test.rb

@@ -298,22 +298,45 @@ class Whitehall::PublishingApiTest < ActiveSupport::TestCase
     assert_requested request
   end
 
-  test ".save_draft raises exception if there is a live content item with the same base path but different content ID" do
+  test ".save_draft raises exception for first new draft, if there is a live content item with the same base path but different content ID" do
     ConfigurableDocumentType.setup_test_types(build_configurable_document_type("test_type"))
-    edition = create(:draft_standard_edition)
+    published_edition = create(:published_standard_edition, title: "Original Title")
+    new_draft_edition_with_same_base_path = create(:draft_standard_edition, title: "Original Title")
+
+    assert new_draft_edition_with_same_base_path.reload.base_path.starts_with?(published_edition.base_path)
+    assert new_draft_edition_with_same_base_path.reload.base_path.ends_with?("--2")
+
     Whitehall::PublishingApi.unstub(:ensure_base_path_is_associated_with_this_content_id!)
-    Services.publishing_api.expects(:lookup_content_id).with(base_path: edition.public_path).returns("different-content-id")
+    Services.publishing_api.expects(:lookup_content_id).with(base_path: published_edition.base_path).returns(published_edition.content_id)
 
     error = assert_raises Whitehall::UnpublishableInstanceError do
-      Whitehall::PublishingApi.save_draft(edition)
+      Whitehall::PublishingApi.save_draft(new_draft_edition_with_same_base_path)
     end
 
     assert_equal(
-      "Cannot save draft (content_id #{edition.content_id}). There is existing content at the '#{edition.public_path}' route, under a different content_id (different-content-id). Try changing your title to resolve the conflict.",
+      "Cannot save draft (content_id #{new_draft_edition_with_same_base_path.content_id}). There is existing content at the '#{published_edition.base_path}' route, under a different content_id (#{published_edition.content_id}). Try changing your title to resolve the conflict.",
       error.message,
     )
   end
 
+  test ".save_draft does no raise an exception if the published path is already sequenced" do
+    ConfigurableDocumentType.setup_test_types(build_configurable_document_type("test_type"))
+    slug = "example-document--2"
+    edition = create(:published_standard_edition)
+    edition.reload.document.update!(slug: slug)
+    draft = edition.create_draft(create(:user))
+
+    assert edition.reload.base_path.ends_with?(slug)
+    assert draft.reload.base_path.ends_with?(slug)
+
+    Whitehall::PublishingApi.unstub(:ensure_base_path_is_associated_with_this_content_id!)
+    Services.publishing_api.expects(:lookup_content_id).with(base_path: draft.base_path).returns(draft.content_id)
+
+    assert_nothing_raised do
+      Whitehall::PublishingApi.save_draft(draft)
+    end
+  end
+
   test ".publish_redirect_async publishes a redirect to the Publishing API" do
     document = create(:document)
     destination = "/government/people/milli-vanilli"