Rails：在验证期间查询关联

Question

During validation, I want to query associations but neither solution seems to be good because ActiveRecord's style of validation. Here is an example:

class User < ApplicationRecord
  has_many :borrowed_books

  validate :should_only_borrow_good_books

  def should_only_borrow_good_books
    # What I want but it does not work:
    #
    # unless borrowed_books.where(condition: "bad").empty?
    #  errors.add(:borrowed_books, "only good books can be borrowed")
    # end
    #
    # ^ this query always returns an empty array

    # This approach works but it's not ideal:
    unless borrowed_books.all? { |b| b.condition == "good" }
      errors.add(:borrowed_books, "only good books can be borrowed")
    end
  end
end

class BorrowedBook < ApplicationRecord
  belongs_to :user
  # attr: condition - ["bad", "good"]
end

One more option is to move the validation to BorrowedBook with something like validates :condition, inclusion: { in: %w(good) }, if: -> { user_id.present? } validates :condition, inclusion: { in: %w(good) }, if: -> { user_id.present? } and perhaps validate association in User like validates_associated :borrowed_books . But I don't like this approach because it complicates things by moving the logic belonging to User to BorrowedBook . A few validations like this and your app might become really messy.

Answer 1

This validation should definitely stay in the user model. I do disagree that it will look messy if there are multiple conditions. If it makes messy, it often indicate that you should split the model or refactor the code there. It's the models job to enable you to access and validate the data from db. A way to improve the current code is to convert the condition column to enum, ref:https://api.rubyonrails.org/v5.1/classes/ActiveRecord/Enum.html you can write it like this

class User < ApplicationRecord
  has_many :borrowed_books

  validate :should_only_borrow_good_books

  private 

  def should_only_borrow_good_books
    return unless books.not_good.any?
      errors.add(:borrowed_books, "only good books can be borrowed")
    end
  end
end

class BorrowedBook < ApplicationRecord
  belongs_to :user
  enum status: [ :good, :bad ]
end