Inject seo translations to prevent IntegrityError.

Fixes django-fluent#47

fluent_pages/models/db.py

@@ -613,7 +613,22 @@ def __str__(self):
     objects = UrlNodeManager()
 
 
-class HtmlPage(Page):
+class InjectSEOTranslationsMetaclass(URLNodeMetaClass):
+    def __new__(cls, name, bases, attrs):
+        # Only assign new attrs if not a proxy model.
+        if not ("Meta" in attrs and getattr(attrs["Meta"], "proxy", False)):
+            field = TranslatedFields(
+                meta={'db_table':'fluent_pages_htmlpage_translation'},
+                meta_keywords=models.CharField(_('keywords'), max_length=255, blank=True, null=True),
+                meta_description=models.CharField(_('description'), max_length=255, blank=True, null=True),
+                meta_title=models.CharField(_('page title'), max_length=255, blank=True, null=True, help_text=_("When this field is not filled in, the menu title text will be used.")),
+            )
+            attrs["seo_translations"] = field
+        args = (cls, name, bases, attrs)
+        return super(InjectSEOTranslationsMetaclass, cls).__new__(*args)
+
+
+class HtmlPage(with_metaclass(InjectSEOTranslationsMetaclass, Page)):
     """
     The base fields for a HTML page of the web site.
 
@@ -626,13 +641,6 @@ class HtmlPage(Page):
     meta_description = TranslatedField()
     meta_title = TranslatedField()
 
-    # SEO fields, the underlying HtmlPageTranslation model can be created dynamically.
-    seo_translations = TranslatedFields(
-        meta_keywords = models.CharField(_('keywords'), max_length=255, blank=True, null=True),
-        meta_description = models.CharField(_('description'), max_length=255, blank=True, null=True),
-        meta_title = models.CharField(_('page title'), max_length=255, blank=True, null=True, help_text=_("When this field is not filled in, the menu title text will be used.")),
-    )
-
     class Meta:
         app_label = 'fluent_pages'
         proxy = True

fluent_pages/tests/modeldata.py

@@ -239,3 +239,16 @@ def test_empty_translation_check(self):
         """
         from fluent_pages.models import UrlNode_Translation
         self.assertRaises(RuntimeError, lambda: UrlNode_Translation.objects.create())
+
+    def test_seo_translations(self):
+        """
+        Seo translations are direct injected onto a concrete model from the
+        HtmlPage proxy model. The field must be visible to the django admin or
+        else there will be an integrity error on deletion.
+        """
+        level2 = SimpleTextPage.objects.get(translations__slug='level2')
+        try:
+            # `get_field_by_name` is deprecated >= django 2.0
+            level2._meta.get_field_by_name('seo_translations')
+        except AttributeError:
+            level2._meta.get_field('seo_translations')