How to perform math calculations in Active Record Querying in Rails - sql

Question

I'm building an application and I'm finding it necessary to perform some simple math calculations in my query. Essentially, I've got a database with daily values from the S&P 500, and I need to get a listing of days depending on the criteria entered.
The user inputs both a day range, and a % range. For instance, if the date range is Jan 1/2013 - Apr 1/2013 and the % range is -1% - 1%, it should return a list of all S%P 500 daily values between the dates where difference between the opening and closing values are in the % range.
The problem is that I don't actually have a column for %; I only have a column for opening/closing values. It is simple enough to calculate the % given only the opening and closing values (close-open)/open*100. But I'm not sure how to do this within the query.
Right now the query is successfully searching within the date range. My query is:
#cases = Close.find(:all, conditions:["date between ? and ?",
#f_start, #f_end])
But how can I get it to check if the current row's (close-open)/open*100 value is between the two % range values?
Alternatively, if this is not possible or in bad practice, where should I be handling this?

Answer 1

You can calculate the open/close range yourself in Ruby/Rails and pass it on the same way you do with the date range. Something like:
percent = #f_percent / 100.0 # 5% => 0.05
low = #f_close * (1.0 - percent)
high = #f_close * (1.0 + percent)
Close.where 'date between ? and ? AND close between ? and ?', \
#f_start, #f_end, low, high