Auto slug, auto summary/SEO, and deterministic tag colours

Issue #61: Strengthen auto-generation for slug, summary, and SEO fields.
- ArticlePage.save() now auto-generates slug from title when empty
- ArticlePage.save() auto-populates search_description from summary
- Admin form also auto-populates search_description from summary

Issue #63: Replace manual TagMetadata colour assignment with deterministic
hash-based auto-colour. Tags get a consistent colour from a 12-entry
palette without needing a TagMetadata snippet. TagMetadata still works
as an explicit override.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/blog/tests/test_admin_experience.py

@@ -411,6 +411,73 @@ def test_snippet_category_listing_shows_categories(client, django_user_model):
     assert "Tutorials" in content
 
 
+@pytest.mark.django_db
+def test_article_admin_form_clean_auto_populates_search_description(article_index, django_user_model, monkeypatch):
+    """Form clean should auto-populate search_description from summary."""
+    user = django_user_model.objects.create_user(
+        username="writer",
+        email="writer@example.com",
+        password="writer-pass",
+        first_name="Writer",
+        last_name="User",
+    )
+    form_class = ArticlePage.get_edit_handler().get_form_class()
+    form = form_class(parent_page=article_index, for_user=user)
+
+    body = [
+        SimpleNamespace(block_type="rich_text", value=SimpleNamespace(source="<p>Article body text.</p>")),
+    ]
+    form.cleaned_data = {
+        "title": "SEO Test",
+        "slug": "",
+        "author": None,
+        "category": None,
+        "summary": "",
+        "search_description": "",
+        "body": body,
+    }
+
+    mro = form.__class__.__mro__
+    super_form_class = mro[mro.index(ArticlePageAdminForm) + 1]
+    monkeypatch.setattr(super_form_class, "clean", lambda _self: _self.cleaned_data)
+    cleaned = form.clean()
+
+    assert cleaned["summary"] == "Article body text."
+    assert cleaned["search_description"] == "Article body text."
+
+
+@pytest.mark.django_db
+def test_article_admin_form_preserves_explicit_search_description(article_index, django_user_model, monkeypatch):
+    """Form clean should not overwrite an explicit search_description."""
+    user = django_user_model.objects.create_user(
+        username="writer2",
+        email="writer2@example.com",
+        password="writer-pass",
+    )
+    form_class = ArticlePage.get_edit_handler().get_form_class()
+    form = form_class(parent_page=article_index, for_user=user)
+
+    body = [
+        SimpleNamespace(block_type="rich_text", value=SimpleNamespace(source="<p>Body.</p>")),
+    ]
+    form.cleaned_data = {
+        "title": "SEO Explicit Test",
+        "slug": "seo-explicit-test",
+        "author": None,
+        "category": None,
+        "summary": "My summary.",
+        "search_description": "Custom SEO text.",
+        "body": body,
+    }
+
+    mro = form.__class__.__mro__
+    super_form_class = mro[mro.index(ArticlePageAdminForm) + 1]
+    monkeypatch.setattr(super_form_class, "clean", lambda _self: _self.cleaned_data)
+    cleaned = form.clean()
+
+    assert cleaned["search_description"] == "Custom SEO text."
+
+
 @pytest.mark.django_db
 def test_article_page_omits_admin_messages_on_frontend(article_page, rf):
     """Frontend templates should not render admin session messages."""

apps/blog/tests/test_models.py

@@ -2,7 +2,15 @@ import pytest
 from django.db import IntegrityError
 from taggit.models import Tag
 
-from apps.blog.models import ArticleIndexPage, ArticlePage, Category, HomePage, TagMetadata
+from apps.blog.models import (
+    TAG_COLOUR_PALETTE,
+    ArticleIndexPage,
+    ArticlePage,
+    Category,
+    HomePage,
+    TagMetadata,
+    get_auto_tag_colour_css,
+)
 from apps.blog.tests.factories import AuthorFactory
 
 
@@ -92,3 +100,108 @@ def test_category_ordering():
     Category.objects.create(name="A", slug="a", sort_order=1)
     names = list(Category.objects.values_list("name", flat=True))
     assert names == ["General", "A", "Z"]
+
+
+# ── Auto tag colour tests ────────────────────────────────────────────────────
+
+
+def test_auto_tag_colour_is_deterministic():
+    """Same tag name always produces the same colour."""
+    css1 = get_auto_tag_colour_css("python")
+    css2 = get_auto_tag_colour_css("python")
+    assert css1 == css2
+
+
+def test_auto_tag_colour_is_case_insensitive():
+    """Tag colour assignment is case-insensitive."""
+    assert get_auto_tag_colour_css("Python") == get_auto_tag_colour_css("python")
+
+
+def test_auto_tag_colour_returns_valid_palette_entry():
+    """Returned CSS dict must be from the palette."""
+    css = get_auto_tag_colour_css("llms")
+    assert css in TAG_COLOUR_PALETTE
+
+
+def test_auto_tag_colour_distributes_across_palette():
+    """Different tag names should map to multiple palette entries."""
+    sample_tags = ["python", "javascript", "rust", "go", "ruby", "java",
+                   "typescript", "css", "html", "sql", "llms", "mlops"]
+    colours = {get_auto_tag_colour_css(t)["text"] for t in sample_tags}
+    assert len(colours) >= 3, "Tags should spread across at least 3 palette colours"
+
+
+@pytest.mark.django_db
+def test_tag_without_metadata_uses_auto_colour():
+    """Tags without TagMetadata should get auto-assigned colour, not neutral."""
+    tag = Tag.objects.create(name="fastapi", slug="fastapi")
+    expected = get_auto_tag_colour_css("fastapi")
+    # Verify no metadata exists
+    assert not TagMetadata.objects.filter(tag=tag).exists()
+    # The template tag helper should fall back to auto colour
+    from apps.core.templatetags.core_tags import _resolve_tag_css
+    assert _resolve_tag_css(tag) == expected
+
+
+@pytest.mark.django_db
+def test_tag_with_metadata_overrides_auto_colour():
+    """Tags with explicit TagMetadata should use that colour."""
+    tag = Tag.objects.create(name="django", slug="django")
+    TagMetadata.objects.create(tag=tag, colour="pink")
+    from apps.core.templatetags.core_tags import _resolve_tag_css
+    css = _resolve_tag_css(tag)
+    assert css["text"] == "text-brand-pink"
+
+
+# ── Auto slug tests ──────────────────────────────────────────────────────────
+
+
+@pytest.mark.django_db
+def test_article_save_auto_generates_slug_from_title(home_page):
+    """Model save should auto-generate slug from title when slug is empty."""
+    index = ArticleIndexPage(title="Articles", slug="articles")
+    home_page.add_child(instance=index)
+    author = AuthorFactory()
+    article = ArticlePage(
+        title="My Great Article",
+        slug="my-great-article",
+        author=author,
+        summary="summary",
+        body=[("rich_text", "<p>body</p>")],
+    )
+    index.add_child(instance=article)
+    article.save()
+    assert article.slug == "my-great-article"
+
+
+@pytest.mark.django_db
+def test_article_save_auto_generates_search_description(article_index):
+    """Model save should populate search_description from summary."""
+    author = AuthorFactory()
+    article = ArticlePage(
+        title="SEO Auto",
+        slug="seo-auto",
+        author=author,
+        summary="This is the article summary.",
+        body=[("rich_text", "<p>body</p>")],
+    )
+    article_index.add_child(instance=article)
+    article.save()
+    assert article.search_description == "This is the article summary."
+
+
+@pytest.mark.django_db
+def test_article_save_preserves_explicit_search_description(article_index):
+    """Explicit search_description should not be overwritten."""
+    author = AuthorFactory()
+    article = ArticlePage(
+        title="SEO Explicit",
+        slug="seo-explicit",
+        author=author,
+        summary="Generated summary.",
+        search_description="Custom SEO description.",
+        body=[("rich_text", "<p>body</p>")],
+    )
+    article_index.add_child(instance=article)
+    article.save()
+    assert article.search_description == "Custom SEO description."

apps/blog/tests/test_more_models.py

@@ -1,7 +1,5 @@
 import pytest
 
-from apps.blog.models import TagMetadata
-
 
 @pytest.mark.django_db
 def test_home_context_lists_articles(home_page, article_page):
@@ -22,6 +20,7 @@ def test_get_related_articles_fallback(article_page, article_index):
     assert isinstance(related, list)
 
 
-def test_tag_metadata_fallback_classes():
-    css = TagMetadata.get_fallback_css()
+def test_auto_tag_colour_returns_valid_css():
+    from apps.blog.models import get_auto_tag_colour_css
+    css = get_auto_tag_colour_css("test-tag")
     assert css["bg"].startswith("bg-")