Question

为了更简单，我有

class User
  has_many :questions, trough: votes
  has_many :questions     #(as the author)
  has_many :votes
end

忘记在创建时添加foreign_key，现在我不知道如何将其添加到特定的（has_many through）关联

schema.rb

  enable_extension "plpgsql"

  create_table "answers", force: :cascade do |t|
    t.text     "body"
    t.datetime "created_at",                  null: false
    t.datetime "updated_at",                  null: false
    t.integer  "question_id"
    t.integer  "user_id"
    t.boolean  "best",        default: false
  end

  add_index "answers", ["question_id"], name: "index_answers_on_question_id", using: :btree
  add_index "answers", ["user_id"], name: "index_answers_on_user_id", using: :btree

  create_table "attachments", force: :cascade do |t|
    t.string   "file"
    t.datetime "created_at",      null: false
    t.datetime "updated_at",      null: false
    t.integer  "attachable_id"
    t.string   "attachable_type"
  end

  add_index "attachments", ["attachable_id", "attachable_type"], name: "index_attachments_on_attachable_id_and_attachable_type", using: :btree

  create_table "questions", force: :cascade do |t|
    t.string   "title"
    t.text     "body"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer  "user_id"
  end

  add_index "questions", ["title"], name: "index_questions_on_title", using: :btree
  add_index "questions", ["user_id"], name: "index_questions_on_user_id", using: :btree

  create_table "users", force: :cascade do |t|
    t.string   "name"
    t.datetime "created_at",                          null: false
    t.datetime "updated_at",                          null: false
    t.string   "email",                  default: "", null: false
    t.string   "encrypted_password",     default: "", null: false
    t.string   "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.integer  "sign_in_count",          default: 0,  null: false
    t.datetime "current_sign_in_at"
    t.datetime "last_sign_in_at"
    t.inet     "current_sign_in_ip"
    t.inet     "last_sign_in_ip"
  end

  add_index "users", ["email"], name: "index_users_on_email", unique: true, using: :btree
  add_index "users", ["name"], name: "index_users_on_name", unique: true, using: :btree
  add_index "users", ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true, using: :btree

  create_table "votes", force: :cascade do |t|
    t.integer  "votable_id"
    t.string   "votable_type"
    t.integer  "user_id"
    t.datetime "created_at",   null: false
    t.datetime "updated_at",   null: false
  end

  add_index "votes", ["user_id", "votable_id"], name: "index_votes_on_user_id_and_votable_id", unique: true, using: :btree

  add_foreign_key "answers", "questions", on_delete: :cascade
  add_foreign_key "questions", "users", on_delete: :cascade
end

Answer 1

在您的控制台上运行此命令

rails g migration AddForeignKeyToVotes user:references question:references

这将在xxxx_add_foreign_key_to_votes.rb下生成db/migrate/个文件，其中包含以下内容

class AddForeignKeyToVotes < ActiveRecord::Migration
  def change
    add_reference :votes, :user, index: true, foreign_key: true
    add_reference :votes, :question, index: true, foreign_key: true
  end
end

Answer 2

你真的需要外键吗？

许多Rails开发人员对Rails处理应用程序而不是数据库中的关系的方式感到满意。

适用于您的情况：

class User
  has_many :questions, trough: votes
  has_many :questions     #(as the author)
  has_many :votes

端

如果votes表有question_id和user_id足以定义没有任何外键的关系，除非你有理由并且确实需要将这个外键定义为数据库级别

仔细阅读THIS SECTION，Rails正在使用Convention over Configuration。

作为一个小例子：您的User模型如何知道要查询哪个表并检索数据，而没有任何配置它搜索具有相同名称users（约定）的表并使用它，相同对于外键。

根据你的评论，你有一个模型的东西，如Stackoverflow，你有一个User谁可以问Question并可以回答Question在这种情况下，你可能有类似的东西：

class User
  has_many :asked_questions, class_name: 'Question'   # user can ask many questions
  has_many :voted_questions, through: :votes, source: 'question'
  has_many :votes # to get all votes this user did
end

class Question
  has_many :votes   # to get all votes for a question
  belongs_to :user
end

class Vote
  belongs_to :user
  belongs_to :question
end

数据库将类似于：

 # Table User
 # Table Question (user_id)
 # Table Vote (user_id, question_id)

我们假设您想让Questions用户问他们会是这样：

user = User.first 
user.asked_questions

如果您想获得用户投票的Questions：

 user.voted_questions

外键轨道，如何添加？

2 个答案: