验证HABTM关联中名称的唯一性

问题描述

我有一个Artist模型，在HABTM关联中有很多Album.尽管我想允许两张不同的专辑使用相同的名称，但我想确保一位艺术家的收藏中没有两张专辑.到目前为止的示例:

I have an Artist model that has many Albums in a HABTM association. Though I would like to permit two different albums to have the same name I'd like to ensure that there aren't two in the collection of one artist. So far example:

artist_1 = Artist.create(name: "Jay-Z")
artist_2 = Artist.create(name: "The Beatles")

album_1 = Album.create(name: "The Black Album", artist_ids: [1])

album_2 = Album.create(name: "The Black Album", artist_ids: [1])
=> Should return error

album_2 = Album.create(name: "The Black Album", artist_ids: [2])
=> Should not return an error

我首先想到了在Album模型中验证名称的唯一性，但是在尝试创建新对象时出现此错误:

I first thought of validating the uniqueness of the name in the Album model but get this error when I try to create a new object:

SQLite3::SQLException: no such column: albums.artist_id: SELECT 1 AS one FROM "albums" WHERE ("albums"."name" = 'The Black Album' AND "albums"."artist_id" IS NULL) LIMIT 1

然后我想到了将验证放入连接模型AlbumArtist中，但出现错误undefined method 'name'(name是相册的属性之一):

I then thought of putting the validation in my join model, AlbumArtist but get the error undefined method 'name' (name is one of the album's attributes):

undefined method `name' for #<AlbumArtist:0x007f8e1fc335e8>

如何使它正常工作?

class Album < ActiveRecord::Base
    has_many :album_artists
    has_many :artist, through: :album_artists
end

class AlbumArtist < ActiveRecord::Base
  belongs_to :album
  belongs_to :artist

  # validates_uniqueness_of :name, scope: [:artist_id]
end

class Artist < ActiveRecord::Base
  has_many :album_artists
  has_many :albums, through: :album_artists

  # validates_uniqueness_of :name, scope: [:artist_id]
end

架构

create_table "albums", force: :cascade do |t|
  t.string   "name"
end

create_table "album_artists", force: :cascade do |t|
  t.integer  "album_id"
  t.integer  "artist_id"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

add_index "album_artists", ["album_id"], name: "index_album_artists_on_album_id"
add_index "album_artists", ["artist_id"], name: "index_album_artists_on_artist_id"

create_table "artists", force: :cascade do |t|
  t.string   "name"
end

推荐答案

在这种情况下，最直接的方法是在关系模型中放置自定义验证器:

The most straightforward way in this case would be to put a custom validator in your relation model:

class AlbumArtist < ActiveRecord::Base
  belongs_to :album
  belongs_to :artist

  validates_presence_of :album, :artist

  validate :ensure_unique, on: :create

  private

  def ensure_unique
    if self.artist.albums.where(name: self.album.name).any?
      errors[:base] << 'Artist already has an album by this name'
    end
  end
end

如果还没有索引，您可能还想在名称列中添加索引.

You may also want to add an index to the name column if you don't already have one.

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