First crack

db/migrate/20251023053837_create_user_groups.rb

@@ -0,0 +1,12 @@
+class CreateUserGroups < ActiveRecord::Migration[8.1]
+  def change
+    create_table :user_groups do |t|
+      t.references :user, null: false, foreign_key: true
+      t.references :group, null: false, foreign_key: true
+
+      t.timestamps
+    end
+
+    add_index :user_groups, [ :user_id, :group_id ], unique: true
+  end
+end