Add upgrade migration

upgrade_migrations/version_1_5_2.rb

@@ -0,0 +1,12 @@
+class Version152UpdateImpressionsTable < ActiveRecord::Migration
+  def self.up
+    add_column :impressions, :params, :text
+
+    add_index :impressions, [:impressionable_type, :impressionable_id, :params], :name => "poly_params_request_index", :unique => false
+  end
+
+  def self.down
+    remove_index :impressions, [:impressionable_type, :impressionable_id, :params], :name => "poly_params_request_index"
+    remove_column :impressions, :params
+  end
+end