参考： https://ruby-china.github.io/rails-guides/active_record_basics.html
重点在于2.1 2.2两部分

2.1 命名约定
默认情况下，Active Record 使用一些命名约定，查找模型和数据库表之间的映射关系。Rails 把模型的类名转换成复数，然后查找对应的数据表。例如，模型类名为 Book，数据表就是 books。Rails 提供的单复数转换功能很强大，常见和不常见的转换方式都能处理。如果类名由多个单词组成，应该按照 Ruby 的约定，使用驼峰式命名法，这时对应的数据库表将使用下划线分隔各单词。因此：

  • 数据库表名：复数，下划线分隔单词（例如 book_clubs）
  • 模型类名：单数，每个单词的首字母大写（例如 BookClub） | 模型/类 | 表/模式 | | :—- | :—- | | Article | articles | | LineItem | line_items | | Deer | deers | | Mouse | mice | | Person | people |

2.2 模式约定
根据字段的作用不同，Active Record 对数据库表中的字段命名也做了相应的约定：

  • 外键：使用 singularized_table_name_id 形式命名，例如 item_id，order_id。创建模型关联后，Active Record 会查找这个字段；
  • 主键：默认情况下，Active Record 使用整数字段 id 作为表的主键。使用 Active Record 迁移创建数据库表时，会自动创建这个字段；

还有一些可选的字段，能为 Active Record 实例添加更多的功能：

  • created_at：创建记录时，自动设为当前的日期和时间；
  • updated_at：更新记录时，自动设为当前的日期和时间；
  • lock_version：在模型中添加乐观锁；
  • type：让模型使用单表继承；
  • (association_name)_type：存储多态关联的类型；
  • (table_name)_count：缓存所关联对象的数量。比如说，一个 Article 有多个 Comment，那么 comments_count 列存储各篇文章现有的评论数量；

ActiveRecord是怎么样表述1-n，1-1，n-n这种形式的

下面用这三个模型来说明
(1-1) 一个学生 student 有一份档案 profile
(1-n) 学校里面有很多班级 teams，每个班级有很多学生 students
(n-n) 学校里有很多老师 teachers，每个老师有很多头衔 titles
这里只做简单模型表述，所以我们用rails g migration 建立迁移文件，及手动建里 model 的方法实现
为了好表述，我们把 1-n左边的这个1对应的表称为左表，n对应的表为右表

1-1关系

> step.1 建模

创建students与profiles的模迁移文件

  1. $ rails g migration create_students_and_profiles
  2.       invoke  active_record
  3.       create    db/migrate/20200328011926_create_students_and_profiles.rb

修改迁移文件内容主体如下

  1. create_table :students do |t|
  2.   t.string :name, comment: '学生姓名'
  3.   t.timestamps
  4. end
  6. create_table :profiles do |t|
  7.   t.references :student
  8.   t.string :library_card_number, comment: '借书证号码'
  9.   t.timestamps
  10. end

同时建立 models/student.rb 与 models/profile.rb 两个模型，如下

  1. models/student.rb
  2. class Student < ApplicationRecord
  3. end
  5. models/profile.rb
  6. class Profile < ApplicationRecord
  7. end

rails db:migrate 后可以看到对应的表结构为

  1.                                         Table "public.students"
  2.    Column   |            Type             | Collation | Nullable |               Default
  3. ------------+-----------------------------+-----------+----------+--------------------------------------
  4.  id         | bigint                      |           | not null | nextval('students_id_seq'::regclass)
  5.  name       | character varying           |           |          |
  6.  created_at | timestamp without time zone |           | not null |
  7.  updated_at | timestamp without time zone |           | not null |
  8. Indexes:
  9.     "students_pkey" PRIMARY KEY, btree (id)
  11.                                              Table "public.profiles"
  12.        Column        |            Type             | Collation | Nullable |               Default
  13. ---------------------+-----------------------------+-----------+----------+--------------------------------------
  14.  id                  | bigint                      |           | not null | nextval('profiles_id_seq'::regclass)
  15.  student_id          | bigint                      |           |          |
  16.  library_card_number | character varying           |           |          |
  17.  created_at          | timestamp without time zone |           | not null |
  18.  updated_at          | timestamp without time zone |           | not null |
  19. Indexes:
  20.     "profiles_pkey" PRIMARY KEY, btree (id)
  21.     "index_profiles_on_student_id" btree (student_id)

> step.2 在模型中声明

1-1的关系，只需要在左表中声明has_one 右表中声明belongs_to即可

  1. class Student < ApplicationRecord
  2.   has_one :profile
  3. end
  5. class Profile < ApplicationRecord
  6.   belongs_to :student
  7. end

打开一个rails console即可测试

  1. $ rails c
  2. irb(main):001:0> Student.create(name: 'ysllyfe')
  3.    (0.2ms)  BEGIN
  4.   Student Create (1.8ms)  INSERT INTO "students" ("name", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["name", "ysllyfe"], ["created_at", "2020-03-28 01:33:21.209412"], ["updated_at", "2020-03-28 01:33:21.209412"]]
  5.    (0.6ms)  COMMIT
  6. => #<Student id: 1, name: "ysllyfe", created_at: "2020-03-28 01:33:21", updated_at: "2020-03-28 01:33:21">
  7. irb(main):002:0> student = Student.first
  8.   Student Load (0.5ms)  SELECT  "students".* FROM "students" ORDER BY "students"."id" ASC LIMIT $1  [["LIMIT", 1]]
  9. => #<Student id: 1, name: "ysllyfe", created_at: "2020-03-28 01:33:21", updated_at: "2020-03-28 01:33:21">
  10. irb(main):003:0> student.profile = Profile.new(library_card_number: 'xxxxxx')
  11.   Profile Load (0.7ms)  SELECT  "profiles".* FROM "profiles" WHERE "profiles"."student_id" = $1 LIMIT $2  [["student_id", 1], ["LIMIT", 1]]
  12.    (0.1ms)  BEGIN
  13.   Profile Create (0.8ms)  INSERT INTO "profiles" ("student_id", "library_card_number", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["student_id", 1], ["library_card_number", "xxxxxx"], ["created_at", "2020-03-28 01:34:14.112116"], ["updated_at", "2020-03-28 01:34:14.112116"]]
  14.    (5.5ms)  COMMIT
  15. => #<Profile id: 1, student_id: 1, library_card_number: "xxxxxx", created_at: "2020-03-28 01:34:14", updated_at: "2020-03-28 01:34:14">

简单的创建student后，只需使用 .profile= 方法，就可以在某个student下建立 1-1 关联的档案
如果我们需要重建这个关联关系，只需要重新调用 .profile= 方法即可

rails5后 belongs_to 声明，强制要求对应的字段有关联值，而默认 .profile= 方法在存在原profile关系时，会把对应的值改为空（默认dependent为nullify），和rails5冲突，我们只需要改为delete或是destroy就可以了

  1. class Student < ApplicationRecord
  2.   has_one :profile, dependent: :destroy
  3. end
  1. irb(main):010:0> student.profile = Profile.new(library_card_number: 'ccc')
  2.   Profile Load (0.5ms)  SELECT  "profiles".* FROM "profiles" WHERE "profiles"."student_id" = $1 LIMIT $2  [["student_id", 1], ["LIMIT", 1]]
  3.    (0.2ms)  BEGIN
  4.   Profile Destroy (0.5ms)  DELETE FROM "profiles" WHERE "profiles"."id" = $1  [["id", 1]]
  5.   Profile Create (0.4ms)  INSERT INTO "profiles" ("student_id", "library_card_number", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"  [["student_id", 1], ["library_card_number", "ccc"], ["created_at", "2020-03-28 01:37:21.835320"], ["updated_at", "2020-03-28 01:37:21.835320"]]
  6.    (0.9ms)  COMMIT
  7. => #<Profile id: 2, student_id: 1, library_card_number: "ccc", created_at: "2020-03-28 01:37:21", updated_at: "2020-03-28 01:37:21">

1-n 关系

> step.1 建模

只需要建新表 teams 及修改 students

rails g migration create_teams

在对应迁移文件里面，修改主体

  1. create_table :teams do |t|
  2.   t.string :name, comment: '班级名称'
  3.   t.timestamps
  4. end
  6. change_table :students do |t|
  7.   t.references :team
  8. end

迁移后对应的表结构为

  1.                                         Table "public.students"
  2.    Column   |            Type             | Collation | Nullable |               Default
  3. ------------+-----------------------------+-----------+----------+--------------------------------------
  4.  id         | bigint                      |           | not null | nextval('students_id_seq'::regclass)
  5.  name       | character varying           |           |          |
  6.  created_at | timestamp without time zone |           | not null |
  7.  updated_at | timestamp without time zone |           | not null |
  8.  team_id    | bigint                      |           |          |
  9. Indexes:
  10.     "students_pkey" PRIMARY KEY, btree (id)
  11.     "index_students_on_team_id" btree (team_id)
  13.                                         Table "public.teams"
  14.    Column   |            Type             | Collation | Nullable |              Default
  15. ------------+-----------------------------+-----------+----------+-----------------------------------
  16.  id         | bigint                      |           | not null | nextval('teams_id_seq'::regclass)
  17.  name       | character varying           |           |          |
  18.  created_at | timestamp without time zone |           | not null |
  19.  updated_at | timestamp without time zone |           | not null |
  20. Indexes:
  21.     "teams_pkey" PRIMARY KEY, btree (id)

>step.2 在模型中声明

  1. class Team < ApplicationRecord
  2.   has_many :students
  3. end
  5. class Student < ApplicationRecord
  6.   has_one :profile, dependent: :destroy
  7.   belongs_to :team
  8. end

对应has_many 也会有 .students= 方法，因为是1-n，.students=[] 后面添加的是数组，即覆盖students为后面的数组，如果是追加，则使用 .students << 方法，后面为追加单个学生

  1. irb(main):015:0> Team.create(name: '班级1')
  2.    (0.1ms)  BEGIN
  3.   Team Create (1.3ms)  INSERT INTO "teams" ("name", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["name", "班级1"], ["created_at", "2020-03-28 01:51:45.409300"], ["updated_at", "2020-03-28 01:51:45.409300"]]
  4.    (0.3ms)  COMMIT
  5. => #<Team id: 1, name: "班级1", created_at: "2020-03-28 01:51:45", updated_at: "2020-03-28 01:51:45">
  6. irb(main):016:0> team = Team.first
  7.   Team Load (0.4ms)  SELECT  "teams".* FROM "teams" ORDER BY "teams"."id" ASC LIMIT $1  [["LIMIT", 1]]
  8. => #<Team id: 1, name: "班级1", created_at: "2020-03-28 01:51:45", updated_at: "2020-03-28 01:51:45">
  9. irb(main):017:0> team.students = Student.all
  10.   Student Load (0.5ms)  SELECT "students".* FROM "students"
  11.   Student Load (0.3ms)  SELECT "students".* FROM "students" WHERE "students"."team_id" = $1  [["team_id", 1]]
  12.    (0.2ms)  BEGIN
  13.   Student Update (6.4ms)  UPDATE "students" SET "team_id" = $1, "updated_at" = $2 WHERE "students"."id" = $3  [["team_id", 1], ["updated_at", "2020-03-28 01:52:16.148332"], ["id", 1]]
  14.    (10.6ms)  COMMIT
  15. => #<ActiveRecord::Relation [#<Student id: 1, name: "ysllyfe", created_at: "2020-03-28 01:33:21", updated_at: "2020-03-28 01:52:16", team_id: 1>]>
  1. irb(main):018:0> team.students << Student.new(name: 'test_student')
  2.    (0.4ms)  BEGIN
  3.   Student Create (1.1ms)  INSERT INTO "students" ("name", "created_at", "updated_at", "team_id") VALUES ($1, $2, $3, $4) RETURNING "id"  [["name", "test_student"], ["created_at", "2020-03-28 01:53:07.508108"], ["updated_at", "2020-03-28 01:53:07.508108"], ["team_id", 1]]
  4.    (1.0ms)  COMMIT
  5. => #<ActiveRecord::Associations::CollectionProxy [#<Student id: 1, name: "ysllyfe", created_at: "2020-03-28 01:33:21", updated_at: "2020-03-28 01:52:16", team_id: 1>, #<Student id: 2, name: "test_student", created_at: "2020-03-28 01:53:07", updated_at: "2020-03-28 01:53:07", team_id: 1>]>

n-n 关系

N-N关系在active_records中很有意思，但是一般很少用，只做简述
N-N的左表和右表都不是强关联，没有像1-1那种，右边依赖于左表存在，如果左表数据删除，右表数据无根的情况
如teachers 与 titles
声明n-n时，只需要建立对应的中间表 teachers_titles (注意顺序)，这个表不需要主键（id），表字段为 teachers_id titles_id
然后在对应的模型 teacher 下声明 has_and_belongs_to_many :titles
在对应的模型 title 下声明 has_and_belongs_to_many :teachers

不需要在models下建立 teachers_title.rb