我正在使用：rails 6.1.4和ruby 3.1.1p18
我已经有一分钟没有在Rails中使用多对多的关联了，我对这个有点迷失。任何帮助/提示将不胜感激。
我很乐意编辑后添加更多的信息，如果要求。
我在rails应用程序living_muay_thai/中创建了一个目录，并试图在其中创建两个模型之间的many to many关系。我有所有的代码。使用console (development)，我尝试在joins表中创建一个记录，我得到一个错误：

3.1.1 :009 > sb.errors.full_messages
 => ["Living muay thai student must exist", "Living muay thai badge must exist"]

我的代码：

generate models/migrations:

rails generate model living_muay_thai/Student
rails generate model living_muay_thai/Badge

现在我不记得我是如何创建连接表的了。我想我尝试了几个不同的生成行，但都以索引名称太长的错误结束，所以我最终手动编辑它以使错误变得愉快。

Migrations:

Students:

class CreateLivingMuayThaiStudents < ActiveRecord::Migration[6.1]
  def change
    create_table :living_muay_thai_students do |t|
      t.string :fname 
      t.string :lname 
      t.timestamps
    end
  end
end

--------------------------
Badges:

class CreateLivingMuayThaiBadges < ActiveRecord::Migration[6.1]
  def up
    create_table :living_muay_thai_badges do |t|
      t.string  :color 
      t.string  :category
      t.integer :number

      t.timestamps
    end
  end

  def down
    drop_table :living_muay_thai_badges
  end
end

------------------
StudentBadges (joins table)

class CreateLivingMuayThaiStudentBadges < ActiveRecord::Migration[6.1]
  def change
    create_table :living_muay_thai_student_badges do |t|
      t.integer :student_id
      t.integer :badge_id

      t.timestamps
    end
    add_index :living_muay_thai_student_badges, :student_id, name: 'students_index'
    add_index :living_muay_thai_student_badges, :badge_id, name: 'badges_index'
  end
end

对于学生徽章迁移，我这样做了（上面），因为这样做的方式一对夫妇的教程提到我得到一个错误... Index name too long...这样就成功了

Models:

Students:

# == Schema Information
#
# Table name: living_muay_thai_students
#
#  id         :bigint           not null, primary key
#  fname      :string
#  lname      :string
#  created_at :datetime         not null
#  updated_at :datetime         not null
#
class LivingMuayThai::Student < ApplicationRecord
    has_many :living_muay_thai_student_badges
    has_many :living_muay_thai_badges, through: :living_muay_thai_student_badges

    has_many :living_muay_thai_levels
end

----------------------
Badges:

# == Schema Information
#
# Table name: living_muay_thai_badges
#
#  id         :bigint           not null, primary key
#  category   :string
#  color      :string
#  number     :integer
#  created_at :datetime         not null
#  updated_at :datetime         not null
#
class LivingMuayThai::Badge < ApplicationRecord
    has_many :living_muay_thai_student_badges
    has_many :living_muay_thai_students, through: :living_muay_thai_student_badges
end

---------------------
StudentBadge

# == Schema Information
#
# Table name: living_muay_thai_student_badges
#
#  id         :bigint           not null, primary key
#  badge_id   :integer
#  student_id :integer
#  created_at :datetime         not null
#  updated_at :datetime         not null
#
# Indexes
#
#  badges_index    (badge_id)
#  students_index  (student_id)
#
class LivingMuayThai::StudentBadge < ApplicationRecord
  belongs_to :living_muay_thai_student
  belongs_to :living_muay_thai_badge
end

我还没把控制器具体化。我想在这之前在控制台上测试一下。我不确定这和我的问题有什么因果关系。（？？）
控制台：

# create a student:
3.1.1 :001 > s1=LivingMuayThai::Student.create(:fname => 'John', :lname=>'Smith')
# works

# create a badge:
3.1.1 :002 > b1=LivingMuayThai::Badge.create(:color => 'White', :category => 'Beginner', :number => 1)
# works

# create record in joins table:
3.1.1 :003 > sb=LivingMuayThai::StudentBadge.create(:student_id => s1.id, :badge_id => b1.id)
 => #<LivingMuayThai::StudentBadge:0x00007f713867a970 id: nil, student_id: 11, badge_id: 140, created_at: nil, updated_at: nil> 
3.1.1 :004 > sb.valid?
 => false 
3.1.1 :005 > sb.errors.full_messages
 => ["Living muay thai student must exist", "Living muay thai badge must exist"]

再次感谢你看这个。非常感谢。