Rails 3 MySQL query question

Question

My Job model has id and percentage fields (among others). percentage may be nil.

I would like to set a default percentage here:

class JobsController
  def new
    ...
    @job.percentage = <what should be here?>
    ...
  end
end

It should be calculated like this:

Take the percentage of the latest job (a job with max id), which percentage isn't nil.
If there are no jobs, or all jobs percentage is nil, it should be 23.

What is the easiest method to calculate this ?

Michelle Tilley · Accepted Answer · 2011-03-04 05:26:55Z

2

You should probably do this in your Job model:

[Update]

Added a test for new_record? in the callback, since percentage can legitimately be nil; we don't want to override it if it's been previously set as such.

class Job < ActiveRecord::Base
  after_initialize :calculate_default_percent

  # other stuff

  private

    def calculate_default_percent
      if self.new_record?
        conditions = { :conditions => "percentage NOT NULL" }
        self.percentage = Job.last(conditions).nil? ? 23 : Job.last(conditions).percentage
      end
    end
end

edited Mar 4, 2011 at 5:26

answered Mar 4, 2011 at 5:10

Michelle Tilley

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Comments

Ashish · Accepted Answer · 2011-03-04 05:32:01Z

1

You have to write a named scope for finding recent record according to your need.

named_scope :recent, :conditions => "percentage != '' && 
             percentage IS NOT NULL", :order => 'ID DESC', :limit => 1

####Before Save

def calculate_default_percent
  record = Job.recent.first
  self.percentage ||= (record.nil? ? 23 : record.percentage)
end

answered Mar 4, 2011 at 5:32

Ashish

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