feat(migrate): add AddIndexOnPostsUserId, down and AddSlugToPosts

examples/blog/db/migrate/20260907000010_add_slug_to_posts.rb

@@ -0,0 +1,41 @@
+class AddSlugToPosts < Prism::Orm::Migration
+  def up
+    # add_column's own default ('') would leave every existing row with the
+    # SAME empty slug, which the unique index below would reject the moment
+    # a second existing post tried to keep it - so every existing row gets
+    # a real, distinct slug backfilled BEFORE the index goes on, the same
+    # slugify Post.create_with_unique_slug uses for new rows (this runs at
+    # the store level, not through the Post model - db:migrate does not
+    # load app/models, see Prism::Cli#db_migrate).
+    add_column :posts, :slug, default: ''
+    backfill_unique_slugs
+    add_index :posts, :slug, unique: true
+  end
+
+  def down
+    remove_index :posts, :slug
+    remove_column :posts, :slug
+  end
+
+  private
+
+  def backfill_unique_slugs
+    seen = {}
+    store.all(:posts).each do |row|
+      base = slugify(row['title'])
+      slug = base
+      suffix = 1
+      while seen.key?(slug)
+        suffix += 1
+        slug = "#{base}-#{suffix}"
+      end
+      seen[slug] = true
+      store.update(:posts, row['id'], 'slug' => slug)
+    end
+  end
+
+  def slugify(title)
+    base = title.to_s.downcase.gsub(/[^a-z0-9]+/, '-').gsub(/\A-+|-+\z/, '')
+    base.empty? ? 'post' : base
+  end
+end