How to store / use a calculated field? - Intermediate Noob -

rubyonrails-talk
1

Hey Everyone

I'm not sure how to ask this properly - that's probably the reason I'm not sure how to implement it. I'm an intermediate noob and I'm not sure how to bring this together...

Here's my scenerio...

people using my system record "readings" every month to determine month to month usage. Here's the calculation...

{ This months reading} - {last months reading} = the usage for this month

When adding a new reading record I want the system to look at the two readings, calculate the usage, and then insert it possibly using an "after_save" function to store the usage. Here's the basic idea, but I'm not sure how to implement it - any ideas or suggestions would be helpful. This seems to be something I would want to handle in my model, but then I'm not sure how I call it when I save the record.

def get_usage(id,lastmonth,reading)

  @last_month=Readings.find(:all, :conditions=[:month => lastmonth, :id=>id])   current_usage =reading-@last_month.reading return current_usage   end

Any help is appreciated.

Thanks

Chris

2

A question, is it necessary to store this in the db at all? I am not sure from your post but it seems that this could be calculated any time it is needed. If that is so then I would suggest, initially at least, not storing it at all. Provide a model method, usage or whatever is appropriate, that looks up the relevant records and returns the value. Then if in the future you decide for efficiency reasons that you must save this in the db it is only a matter of storing it as appropriate and changing your usage method to return the saved value direct from the db. One of the issues with saving it is that if you later edit one of the records that it is calculated from you must remember to update the saved value.

Colin