return custom query select in activerecord

问题

I've got a query that does some math and returns a calculated custom select field with the result set. I cannot figure out how to access that in the activerecord object that is returned. I've added an attr_accessor for it also.

attr_accessor :percentage_used

select('gateways.*, (num_transactions_today/ SUM(num_transactions_today)) AS percentage_used ').joins(:gateway_groups).where('map_gateway_groups.gateway_group_id = ?', gateway_group_id)

in the result set, I would expect to have access to :percentage_used, but it is not in there. Any ideas on what i'm doing wrong? i've never needed to do this before.

Thanks

回答1:

You can access it as

object["percentage_used"]

回答2:

You neither need nor want attr_accessor for that. attr_accessor creates an instance variable, an accessor method for getting the value of that instance variable, and a mutator method for changing its value. When you say this:

select('gateways.*, (num_transactions_today/ SUM(num_transactions_today)) AS percentage_used ...

ActiveRecord will automatically add a percentage_used method to the returned objects. But the percentage_used method for accessing that value will be added by method_missing. Since you've said attr_accessor :percentage_used, method_missing will never be called and you can't get at the percentage_used value from the query in the usual way.

If you drop the attr_accessor :percentage_used, then you'll be able to call percentage_used on objects returned by that select and you'll find the values you're looking for. However, AR won't be able to convert the value to a native Ruby number though so you'll have to to_f the returned string yourself.

来源：https://stackoverflow.com/questions/18452772/return-custom-query-select-in-activerecord