Ruby on Rails：如果表值在指定范围之间，则返回名称

Question

I have three tables (that are relevant to this problem). One table is called organizations.

I also have a table called organization_details, which contains organization_id and multi-row information about the organization.

I work in the event industry, so the organization_details table contains a column called total_attendance, where a person can input an integer of the org's attendance for a certain year.

The third table is called divisions. This has five rows total, with columns division_smallest and division_largest (referring to the attendance range). Each row has a range to separate which division an organization should belong to according to their most recent attendance record.

For example, one row in the division table shows a division_smallest equal to 1 and a division_largest equal to 100000 (again, referring to attendance). Finally, the division table also has a name column (eg "Division 1").

I want the app to automatically figure out which division an organization belongs to according to their most recent total_attendance. Ideally, the division's name would display in the organization index and show pages.

I'd like to make a custom method for this, but am unsure how best to tackle it. I've read a little bit about .between? as in (possibly) .between?(division.division_smallest, division.division_largest) return "#{division.name}"

...But I am not sure how the entire method would work or if I need to steer away from that entirely. I would greatly appreciate any insight into this!

Answer 1

My suggestion is to add the following method to organization.rb

def division_name
  last_details = organization_details.order('created_at DESC').first
  if last_details.present?
    Division.where(':attendance >= division_smallest AND :attendance <= division_largest', attendance: last_details.attendance).first.name
  else
    "None"
  end
end

The code first grabs the organization details that have been created most recently. If the organization has organization details it uses the attendance value to select the appropriate division and it returns that division's name. If the organization doesn't have any organization_details it returns the string "None". You may also want to handle the case where the attendance isn't inside of the range on any of the divisions you have defined.

I hope this points you in the right direction.

Answer 2

A naive implementation might look something like this:

class Division
  def self.for_attendance(total)
    first('? BETWEEN divisions.division_smallest AND divisions.division_largest', total)
  end
end

class Organization
  def latest_division
    Division.for_attendance(organization_details.last.try(:total_attendance))
  end
end

Now calling some_organization.latest_division will pull the latest division for that organization . This is great for a 'show' page, but will run you into trouble when you have an 'index' with many Organizations - these 2 queries will need to run for each Organization (an N+1 problem). Instead use this:

class Division
  def self.merge_latest!(organizations)
    left_join = "LEFT JOIN organization_details od2 ON organization_details.organization_id = od2.organization_id AND organization_details.created_at < od2.created_at"
    subquery = OrganizationDetails.where(organization_id: organizations.map(&:id)).
       joins(left_join).
       where(od2: {id: nil}).to_sql
    divisions = joins("#{subquery} as t ON t.total_attendance  divisions.division_smallest AND divisions.division_largest").
                select('divisions.*, t.organization_id')
    organizations.each {|org| org.latest_division = divisions.detect{|d| d.organization_id == org.id}
  end
end

def Organization
  attr_accessor :latest_division
end

Now you can call Division.merge_latest!(organizations) to collect the latest division for all the organizations in a single query, addressable via an organization's :latest_division attribute.