sort_by并不总是存在的值– ActiveRecord | 滑轨5

Question

I'm working on a listing site that works with Location and Review models. I was to be able to order the list of locations by their average rating calculated by any reviews that exist for the location.

Location.all.sort_by(&:rating)

throws the error:

An ArgumentError occurred in search#search:

  comparison of BigDecimal with nil failed

Notice rating is a model method, not a field in the database, hence the use of sort_by .
How should I best modify to get this going?
Any help would be much appreciated!

class Location < ApplicationRecord
  has_many :reviews

  def rating
    self.reviews.average('rating')
  end
end

class Review < ApplicationRecord
  belongs_to :location

  validates :title, presence: true
  validates :email, presence: true
  validates :rating, presence: true
  validates :comment, length: { maximum: 500 }
  validates :location, presence: true
end

Answer 1

To avoid having to overwrite your rating method (you might want it to return nil if there are no reviews), you can use the following:

Location.all.sort_by { |l| l.rating || 0 }

This will allow any location without a rating to fallback to a default of 0 in the ordering only if it's not got a rating.

Then, for instance, you could still distinguish between locations with a genuine review score of 0 and those without any reviews.

If you're concerned about genuine scores of 0 clashing with those with not reviews, you can set the default lower, ie -1 .

For example, you might have somewhere you want to display review scores as follows:

= @location.rating || "Location has no reviews"

Answer 2

For example you can return 0 if no review are available:

  def rating
    return 0 unless reviews.any?
    reviews.average(:rating)
  end