ActiveRecord 多对多我应该制作另一个 table 来保存关系信息吗？

Question

我有 albums 和 artists table 应该尽可能多地相关：每个专辑可以由多个艺术家拥有，每个艺术家可以有多个专辑。但是我需要知道画师是主要画师还是贡献者。

目前我只使用 albums table 中的 artists 列将艺术家 ID 保存在分号分隔的字符串中（格式：3;6;343;56;1）。主要艺术家的 id 应该首先出现，其余的只是贡献者。

目前，我通过使用 .where 和 LIKE 关键字查询访问艺术家在专辑中的贡献。然后根据 this answer.

过滤数组结果以排除前缀（艺术家 ID）

@albums_ = Album.where("artists LIKE :prefix", prefix: "%#{params[:id]}%")
@albums_contribute_to = @albums_.select { |album| !album.artists.start_with?("#{params[:id]}") }

有没有更有效的方法来做到这一点？

Answer 1

使用连接table是典型的情况。如果你想建立那种关系。你正在做的事情只会在很长的运行中产生问题，因为你必须将这些信息保存在内存中，这也很慢。

在 ruby 和 rails 中，该关系需要以下内容：

class Album < ApplicationRecord
  has_many :album_artists
  has_many :artists, through: :album_artists
end

class AlbumArtist < ApplicationRecord
  belongs_to :artist
  belongs_to :album
end

class Artist < ApplicationRecord
  has_many :album_artists
  has_many :albums, through: :album_artists
end

如果您想在 rails 上使用 ruby 避免这样做，您必须实施自定义查询，但规则是相同的，您必须创建某种连接 table.

Answer 2

我真的不认为您需要一个单独的模型来加入 Album 和 Artist 模型。因此，您可以使用来自 ActiveRecord 并用于 传递多对多 关系的 has_and_belong_to_many(HABTM) 方法。您还需要一个 table 来连接这两个模型，您可以通过迁移轻松创建一个。

我强烈建议您阅读这篇文章 article。这对你真的很有帮助。

Answer 3

试了一下，主要的想法是在连接 table 上添加一些附加信息（在我的示例中，我称之为 primary）。然后阅读 docs 了解如何告诉 ActiveRecord 使用它们。

# setup active record
require 'active_record'
ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'

# some tables (migrations / schema)
ActiveRecord::Schema.define do
  create_table(:artists) { |t| t.string :name}
  create_table(:albums)  { |t| t.string :name }
  create_table :appearances do |t|
    t.integer :artist_id
    t.integer :album_id
    t.boolean :primary, default: false # <-- join table specifies who is primary
  end
end

# some models
class Artist < ActiveRecord::Base
  has_many :appearances
  has_many :albums, through: :appearances
end   

class Appearance < ActiveRecord::Base
  belongs_to :artist
  belongs_to :album
end

class Album < ActiveRecord::Base
  # three associations to appearances
  has_many :all_appearances,                                   class_name: 'Appearance'
  has_many :primary_appearances,  -> { where primary: true  }, class_name: 'Appearance'
  has_many :featured_appearances, -> { where primary: false }, class_name: 'Appearance'

  # three associations to artists
  has_many :all_artists,      through: 'all_appearances',      source: 'artist'
  has_many :primary_artists,  through: 'primary_appearances',  source: 'artist'
  has_many :featured_artists, through: 'featured_appearances', source: 'artist'
end

# some artists
dre  = Artist.create! name: 'Dr. Dre'
dogg = Artist.create! name: 'Snoop Dogg'
slim = Artist.create! name: 'Eminem'

# some albums
weed = Album.create! name: 'The Chronic 2001',
                     primary_artists:  [dre],
                     featured_artists: [dogg, slim]

show = Album.create! name: 'The Eminem Show',
                     primary_artists:  [slim],
                     featured_artists: [dre]

# it understands the associations
weed.all_artists.pluck :name      # => ["Dr. Dre", "Snoop Dogg", "Eminem"]
weed.primary_artists.pluck :name  # => ["Dr. Dre"]
weed.featured_artists.pluck :name # => ["Snoop Dogg", "Eminem"]

# might be nice to add similar scoped associations to get from Artist to Album
weed               # => #<Album id: 1, name: "The Chronic 2001">
  .primary_artists # => #<ActiveRecord::Associations::CollectionProxy [#<Artist id: 1, name: "Dr. Dre">]>
  .first           # => #<Artist id: 1, name: "Dr. Dre">
  .albums          # => #<ActiveRecord::Associations::CollectionProxy [#<Album id: 1, name: "The Chronic 2001">, #<Album id: 2, name: "The Eminem Show">]>
  .last            # => #<Album id: 2, name: "The Eminem Show">
  .primary_artists # => #<ActiveRecord::Associations::CollectionProxy [#<Artist id: 3, name: "Eminem">]>
  .first           # => #<Artist id: 3, name: "Eminem">

Answer 4

同意 Joshua 和 Przemek 的观点，这是一个简单的解决方案。下面是如何使用它：

class Appearance < ActiveRecord::Base
  belongs_to :artist
  belongs_to :album
  # has attribute "primary"
end

class Artist < ActiveRecord::Base
  has_many :appearances
  has_many :albums, through: :appearances
end

class Album < ActiveRecord::Base
  has_many :appearances
  has_many :artists, through: :appearances
end

# create album with 2 artists
Album.create(
  name: "foo_bar",
  appearances: [
    Appearance.new(artist: Artist.create(name: "foo"), primary: true),
    Appearance.new(artist: Artist.create(name: "bar"))
  ]
)

# list artists
album = Album.first
album.appearances.order("piramary desc").each do |appearance|
  puts "#{appearance.artist.name} #{appearance.primary? ? "(Primary)" : ""}"
end

ActiveRecord 多对多我应该制作另一个 table 来保存关系信息吗？

ActiveRecord many to many should I make another table to hold the relation information?

ruby

sql

postgresql

rdbms

ruby-on-rails