Rails 多对多 return 条没有关联的记录

Question

我在 Domains 和 Families

之间存在多对多关系

class Family < ActiveRecord::Base
  has_many :domains_families
  has_many :domains, through: :domains_families
end

class DomainsFamily < ActiveRecord::Base
  belongs_to :family
  belongs_to :domain
end

class Domain < ActiveRecord::Base
  has_many :domains_families
  has_many :families, through: :domains_families
end

我想得到所有 families 没有 domains

我创建了这个方法，它有效，但我知道它可以更有效地完成

def self.no_domains
  families = []

 all.each do |family|
   families << family if family.domains.blank?
 end

 families
end

完成此任务的正确方法是什么？

Answer 1

可能重复：Rails 4 scope to find parents with no children

scope :without_children, includes(:children).where(:children => { :id => nil })

你的情况：

Family.includes(:domains).where(domains: { id: nil })

Rails 多对多 return 条没有关联的记录

Rails many to many return records with no association

ruby-on-rails

postgresql

model-associations