我收到此错误

PG::UndefinedColumn: ERROR:  column cases.invoice_id does not exist
LINE 1: SELECT COUNT(*) FROM "cases" WHERE ("cases"."invoice_id" IS ...
                                            ^
: SELECT COUNT(*) FROM "cases" WHERE ("cases"."invoice_id" IS NOT NULL)

我试图计数所有的发票，我做这个代码

All the Invoice (<%= Case.where.not(invoice: nil).count %>)</h3>

我怎样才能让它工作呢？

class AddAttachmentInvoiceToCases < ActiveRecord::Migration
  def self.up
    change_table :cases do |t|
      t.attachment :invoice
    end
  end

  def self.down
    remove_attachment :cases, :invoice
  end
end

<%= link_to "Your invoice link", item.invoice.url, target: "_blank" %> <% if item.created_at.present? %>