验证 HABTM 关联中名称的唯一性

Question

我有一个 Artist 模型，它在 HABTM 关联中有许多 Albums。虽然我想允许两张不同的专辑具有相同的名称，但我想确保一位艺术家的收藏中没有两张。到目前为止的例子：

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

我最初想在Album 模型中验证名称的唯一性，但在尝试创建新对象时出现此错误：

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

然后我想将验证放入我的连接模型AlbumArtist，但收到错误undefined method 'name'（name 是专辑的属性之一）：

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

我怎样才能让它工作？

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

Answer 1

在这种情况下，最直接的方法是在您的关系模型中放置一个自定义验证器：

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

如果您还没有索引，您可能还想为 name 列添加索引。