ruby-on-rails - belongs_to关联在实例化时发生冲突， "Validation failed... must exist"错误

Question

这就是我进行关联的方式:

class Event < ApplicationRecord
  has_one :lineup
  has_many :artists, :through => :lineup
  belongs_to :venue
end

和

class Lineup < ApplicationRecord
  belongs_to :artist
  belongs_to :event
end

这就是我尝试播种的方式

Event.create!(name: "The Function", 
              date: DateTime.new(2016,2,3,10,0,0,'+7'), 
              venue: Venue.create!(name: "Speakeasy", address: "Lynwood Ave", zip_code: "30312"), 
              lineup: Lineup.create!(:artist => Artist.create!(name: "DJ Sliink", bio: "jersey club king")), 
              description: "free free free")

我得到的错误是 ActiveRecord::RecordInvalid: Validation failed: Event must exist 指向 lineup 行。如果我设置 lineup: nil，我会收到相同的消息，然后在创建 Event 后，尝试执行 event1.lineup = Lineup.create!... 。我有什么选择可以摆脱 Lineup 对现有事件的依赖？据我了解，问题出在 belongs_to 关系上，因为如果我从 Lineup 实例创建中取出 artist: ，我也会得到 Artist must存在错误。

验证:

> Event.validators
 => [#<ActiveRecord::Validations::PresenceValidator:0x007fcaab69fa78 @attributes=[:venue], @options={:message=>:required}>]

> Lineup.validators
 => [#<ActiveRecord::Validations::PresenceValidator:0x007fcaad988238 @attributes=[:artist], @options={:message=>:required}>, #<ActiveRecord::Validations::PresenceValidator:0x007fcaab77c7c0 @attributes=[:event], @options={:message=>:required}>]

如果种子文件中没有 !，我会得到一个完全不同的错误 - ActiveRecord::AssociationTypeMismatch: Venue(#70094220768860) expected, got Fixnum(#70094214808600) ，虽然仍然是 event = Event.create 行。

Answer 1

在 Rails 5 中 belong_to 默认充当 validates_presence_of 所以为了允许一个不存在的关系添加 optional: true 选项到属于_to。例如:

belongs_to :parent,
           optional: true