您有一个多对多关系，这意味着我们需要一个连接表Recommendation。
使用生成器创建模型和迁移文件：

bin/rails generate model Movie
bin/rails generate model Recommendation

然后更新迁移：

# db/migrate/20221023063944_create_movies.rb
class CreateMovies < ActiveRecord::Migration[7.0]
  def change
    create_table :movies do |t|
      # TODO: add fields
    end
  end
end

# db/migrate/20221023064241_create_recommendations.rb
class CreateRecommendations < ActiveRecord::Migration[7.0]
  def change
    create_table :recommendations do |t|
      t.references :movie,             null: false, foreign_key: true
      t.references :recommended_movie, null: false, foreign_key: { to_table: :movies }
    end
  end
end

运行迁移：

bin/rails db:migrate

设置模型：

# app/models/movie.rb
class Movie < ApplicationRecord
  # NOTE: this is the relationship for join table
  has_many :recommendations, dependent: :destroy

  # NOTE: get movies from join table
  has_many :recommended_movies, through: :recommendations
  #       this ^ is the name of the relationship in `Recommendation` we want
end

# app/models/recommendation.rb
class Recommendation < ApplicationRecord
  belongs_to :movie

  # NOTE: our foreign key is `recommended_movie_id` which rails infers 
  #       from `:recommended_movie`, but we have to specify the class:
  belongs_to :recommended_movie, class_name: "Movie"
end

在控制台bin/rails console中进行测试：

>> 3.times { Movie.create }
>> Movie.first.recommended_movies << [Movie.second, Movie.third]
>> Movie.first.recommended_movies
=> [#<Movie:0x00007f15802ec4c0 id: 2>, #<Movie:0x00007f15802ec3d0 id: 3>]

或类似于：

>> Movie.second.recommendations << Recommendation.new(recommended_movie: Movie.first)
>> Movie.second.recommended_movies
=> [#<Movie:0x00007f158215ef20 id: 1>]

• 网址：guides.rubyonrails.org/association_basics.html#the-has-many-through-association*
• 网址：guides.rubyonrails.org/association_basics.html#self-joins*

创建迁移时，需要定义要分配的模型引用

create_table :student do |t|
 t.references :class, foreign_key: true
end

这里我告诉我的class表将student的主键存储为外键。迁移后，class中将有一个名为student_id的列，它存储student表的pk。然后，我将在class模型文件中定义一个关联

class student < ApplicationRecord
  belongs_to :class
end

这将有助于我查询，以便我可以编写

student= Student.find 'student_id'
class = student.class

这将返回该学生的类。对于has_many，过程是相同的，但它将返回数组