验证HABTM协会中名称的唯一性

Question

我有一个Artist模型，该模型在HABTM关联中有很多Album s。虽然我想允许两张不同的专辑具有相同的名称，但我希望确保一位艺术家的收藏中没有两张专辑。到目前为止，例如：

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 

我首先想到的验证名称的唯一的Album模型，但是当我尝试创建一个新的对象得到这个错误：

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 

当时我就想当头在我加盟的模式，AlbumArtist但得到的错误undefined method 'name'验证（name是这张专辑的属性之一）：

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

我怎样才能使这个工作？

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 

模式

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，最简单的方法是把一个自定义验证你的关系模型：

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 

您可能还需要增加一个如果您还没有名称列的索引。