ActiveRecord::HasManyThroughOrderError: 不能有 has_many :through association

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

当我尝试为用户添加徽章时：

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

User.last.badges << b

我得到这个错误：

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

还有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

关联再添加

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，如果你第一次错误地写了 has_many 2 次，那么它也会重现这个错误。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......

Answer 3

在将 Rails 应用程序从 5.2 迁移到 6 时，我遇到了这个错误，但在我的例子中，关系顺序是正确的：

has_many :users_roles
has_many :roles,      :through => :users_roles

[ActiveRecord::HasManyThroughOrderError: Cannot have a has_many :through association](https://stackoverflow.com/questions/49450963/activerecordhasmanythroughordererror-cannot-have-a-has-many-through-associat)

该应用程序使用

rolify

并且它在班级中名列前茅：

我不得不改变顺序：

has_many :users_roles
----------
rolify
----------
has_many :roles,      :through => :users_roles