Question

Just wondering if there is any function that allows to do that:

MyModel.find_by_conditon(method_a - method_b > 0)

MyModel.class

def method_a
  next_service_date.to_i
end

def method_b
  last_service_date.to_i
end

def next_service_date
  service_date.present? ? calculated_time_estimation : row_time_estimation
end
def calculated_time_estimation
  service_date + calculated_service_period
end
def calculated_service_period
      case(service_frequency_period)
      when 'Year'
        service_frequency_number.to_i.year
      when 'Month'
        service_frequency_number.to_i.month
      when 'Week'
        service_frequency_number.to_i.day * 7
      when 'Day'
        service_frequency_number.to_i.day
      end
end

service_frequency_number, service_frequency_period, service_date are attributes for MyModel

Answer 1

Assuming that method_a and method_b are actually attributes, you can do:

MyModel.where('method_a - method_b > ?', 0)

Edit

I think the only way that you’re going to be able to query on a calculated field is to move the calculation to the DB, where you would have a scalar function to return the results. Then you could do MyModel.where('next_service_date_on_db(service_date) > 0'). Unfortunately, that would tie you to a DB-specific implementation, but you’re not going to be able to query that way without a server-side function.

Now, if you have the entire collection, you could filter based on those conditions, but you’ll have to load the entire set. For example:

MyModel.all.select {|m| m.method_a - m.method_b > 0}

#all returns an array of all of the objects, and select filters based on the condition of the block. Unfortunately, this solution loads all of the records to sort application side.