Rails：使用自定义方法订购自定义模型

Question

I have a custom model called product, and it has many reviews.

i have a method that calculates the review

def rating
    total = 0
    reviews_count = reviews.count

    return 0 if reviews_count == 0

    reviews.each do |review|
        total += review.grade
    end

    total.to_f/reviews_count
end

i would like to know how could i use this method to Order my products.

At products_controller.rb, if i use:

@products = Product.all.order("price")

its easy, it gives me the products list ordered by price. But, if i use, for example:

@products = Product.all.sort_by{|p| p.rating}

it gives me an array and not a "ActiveRecord::Relation"

I would like to know how could i order my product using a custom method that returns a value.

Answer 1

In general, you can't. Ordering happens in your database, which has no knowledge about any method you typed in your application. What you need is a way of translating your method into a valid sql. In your case, you can do:

Product.joins(:reviews).group('products.id').order('AVG(reviews.grade)')

That will give you sorted results and the relation object. However, relations with join are not that nice to work with, especially if you try to add another join. Also this might get quite slow when your database grows.

Answer 2

What you're doing in your example is running the query then using sort_by to sort the result set.

If you want to get back an activerecord collection instead of an array, and potentially chain this with other scopes, you should move the logic from your method into SQL, and put it in a scope.