Supplying migrations for a bold new era of mediagoblin entry slugs

Okay, that's a totally confusing statement, but the docstring of this
migration summarizes it well:

  Entries without slugs now display differently in the url like:
    /u/cwebber/m/id=251/

  ... because of this, we should back-convert:
   - entries without slugs should be converted to use the id, if possible, to
     make old urls still work
   - slugs with = (or also : which is now also not allowed) to have those
     stripped out (small possibility of breakage here sadly)

This commit sponsored by John Sullivan.  Thanks johnsu01! :)

mediagoblin/db/migrations.py

@@ -195,3 +195,57 @@ def add_license_preference(db):
     col = Column('license_preference', Unicode)
     col.create(user_table)
     db.commit()
+
+
+@RegisterMigration(9, MIGRATIONS)
+def mediaentry_new_slug_era(db):
+    """
+    Update for the new era for media type slugs.
+
+    Entries without slugs now display differently in the url like:
+      /u/cwebber/m/id=251/
+
+    ... because of this, we should back-convert:
+     - entries without slugs should be converted to use the id, if possible, to
+       make old urls still work
+     - slugs with = (or also : which is now also not allowed) to have those
+       stripped out (small possibility of breakage here sadly)
+    """
+    import uuid
+
+    def slug_and_user_combo_exists(slug, uploader):
+        # Technically returns the number of entries with this slug and user
+        # that already exist
+        return db.execute(
+            media_table.select(
+                media_table.c.uploader==uploader,
+                media_table.c.slug==slug)).first().tbl_row_count
+
+    def append_garbage_till_unique(row, new_slug):
+        """
+        Attach junk to this row until it's unique, then save it
+        """
+        if slug_and_user_combo_exists(new_slug, row.uploader):
+            # okay, still no success;
+            # let's whack junk on there till it's unique.
+            new_slug += '-' + uuid.uuid4().hex[:4]
+            # keep going if necessary!
+            while slug_and_user_combo_exists(new_slug, row.uploader):
+                new_slug += uuid.uuid4().hex[:4]
+
+        db.execute(
+            media_table.update(). \
+            where(media_table.c.id==row.id). \
+            values(slug=new_slug))
+
+    metadata = MetaData(bind=db.bind)
+
+    media_table = inspect_table(metadata, 'core__media_entries')
+    for row in db.execute(media_table.select()):
+        # no slug, try setting to an id
+        if not row.slug:
+            append_garbage_till_unique(row, unicode(row.id))
+        # has "=" or ":" in it... we're getting rid of those
+        elif u"=" in row.slug or u":" in row.slug:
+            append_garbage_till_unique(
+                row, row.slug.replace(u"=", u"-").replace(u":", u"-"))