correct way to find max value with association rails

问题

I have the following models:

#equipment.rb
class Equipment < ActiveRecord::Base
  belongs_to :odometer_type
  has_many :odometers
end

#odometer.rb
class Odometer < ActiveRecord::Base
  belongs_to :equipment

  # I am currently doing this to find the last mileage entered (which is wrong too cause I want the mileage from the max date entered)

  def self.current_reading(equipment)
    all.where(:equipment_id => equipment.id).max(:mileage)
  end  
end

This looks even worse in the view, like this:

= @equipment.odometers.current_reading(@equipment)

I am thinking there should be a way better way to do this, but I can't seem to come up with or find anything. I am honestly not even sure how to search for something like this.

Thanks for any help.

回答1:

If you want the mileage for the last inserted odometer of an equipment you do

# assuming autoincrement id
@equipment.odometers.order('odometers.id DESC').limit(1).first.mileage

# assuming created_at column
@equipment.odometers.order('odometers.created_at DESC').limit(1).first.mileage

If you want to take the max odometer mileage for an equipment:

@equipment.odometers.maximum(:mileage)

Because of the Equipment has_many :odometers relation, in the above code the :equipment_id => equipment.id condition is implicit.

You may want to implement something similar to a counter cache, only that insted of taking the count you take the max, to speed up queries.

回答2:

You can put this in your equipment model

class Equipment < ActiveRecord::Base

  belongs_to :odometer_type
  has_many :odometers

  def max_milage
    odometers.max(:mileage)
  end
end

Then you can access it like @equipment.max_milage

来源：https://stackoverflow.com/questions/8567175/correct-way-to-find-max-value-with-association-rails