title: Is there a way to add to an existing balance or amount in a database column without just replacing it when updating?
body: <p>I am building a budgeting app of sorts. So I need to be able to make a deposit and a withdrawal. However the update method replaces the previous amount rather than adding to it. When I google this question I get active record migrations. Is there something similar to update_attributes that adds on to rather than replacing? Or is there a db column type that works this way? 
  This is what I have right now. </p>
  
  <pre><code>    def deposit
       account = find_account(params[:id])
       new_balance = account.balance += params[:account][:balance].to_f
       account.assign_attributes(balance: new_balance)
       if account.save
        redirect_to accounts_path
       else
        redirect_to :back
      end
    end
  </code></pre>