ActiveRecord::HasManyThroughOrderError: 不能有 has_many :through 关联

Question

在我的 rails 应用程序中，我正在尝试创建一个系统，该系统将奖励用户获得各种成就的徽章

创建了一个表'user_badges'

迁移：

class CreateUserBadges < ActiveRecord::Migration[5.1]
  def change
    create_table :user_badges do |t|

    t.references :user, foreign_key: true
    t.references :badge, foreign_key: true

    t.timestamps
    end
  end
end

模型用户徽章：

class UserBadge < ApplicationRecord

  belongs_to :user
  belongs_to :badge

end

модель 徽章：

class Badge < ApplicationRecord
  has_many :users, through: :user_badges
  has_many :user_badges
end

模型用户：

class User < ApplicationRecord
  ...

  has_many :badges, through: :user_badges
  has_many :user_badges

  ...
end

当我尝试向用户添加徽章时：

b = Badge.create(title: 'first')

User.last.badges << b

我收到此错误：

ActiveRecord::HasManyThroughOrderError: Cannot have a has_many 
:through association 'User#badges' which goes through 
'User#user_badges' before the through association is defined.

当我简单地打电话时：

User.last.badges

同样的错误：

ActiveRecord::HasManyThroughOrderError: Cannot have a has_many 
:through association 'User#badges' which goes through 
'User#user_badges' before the through association is defined.

Answer 1

先定义has_many关联，然后添加through:关联

class UserBadge < ApplicationRecord
  belongs_to :user
  belongs_to :badge
end

class Badge < ApplicationRecord
  has_many :user_badges # has_many association comes first
  has_many :users, through: :user_badges #through association comes after
end


class User < ApplicationRecord
  ...
  has_many :user_badges
  has_many :badges, through: :user_badges
  ...
end

Answer 2

注意，如果你错误地先写了两次has_many，那么它也可以重现这个错误。例如

class User < ApplicationRecord
  ...
  has_many :user_badges
  has_many :badges, through: :user_badges
  ...
  has_many :user_badges
end

# => Leads to the error of ActiveRecord::HasManyThroughOrderError: Cannot have a has_many :through association 'User#badges' which goes through 'User#user_badges' before the through association is defined.

Active Record 应该提醒 has_many 被使用了两次恕我直言...