第6篇：ActiveRecord 关联关系

class Post < ApplicationRecord
  belongs_to :user
end

# 生成的方法：
post.user          # 获取关联的user
post.user = user   # 设置关联
post.build_user    # 构建新user
post.create_user   # 创建并保存user
post.reload_user   # 重新加载

create_table :posts do |t|
  t.references :user, foreign_key: true
end

class User < ApplicationRecord
  has_one :profile
end

# 生成方法与 belongs_to 类似
user.profile
user.profile = profile
user.build_profile
user.create_profile

class User < ApplicationRecord
  has_many :posts
end

user.posts                  # 获取所有posts
user.posts << post          # 添加post
user.posts.build            # 构建新post
user.posts.create           # 创建并保存
user.posts.count            # 计数
user.posts.clear            # 清空
user.posts.destroy_all      # 删除所有

class Physician < ApplicationRecord
  has_many :appointments
  has_many :patients, through: :appointments
end

class Appointment < ApplicationRecord
  belongs_to :physician
  belongs_to :patient
end

class Patient < ApplicationRecord
  has_many :appointments
  has_many :physicians, through: :appointments
end

class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true
end

class Post < ApplicationRecord
  has_many :comments, as: :commentable
end

class Photo < ApplicationRecord
  has_many :comments, as: :commentable
end

# 数据库结构
create_table :comments do |t|
  t.references :commentable, polymorphic: true
end

class User < ApplicationRecord
  has_many :posts,
    dependent: :destroy,      # 删除策略
    foreign_key: 'author_id', # 外键名
    class_name: 'Article',    # 关联类名
    inverse_of: :author,      # 反向关联
    counter_cache: true,      # 计数缓存
    touch: true              # 更新时间戳
end

has_many :posts, dependent: :destroy      # 删除关联记录
has_many :posts, dependent: :delete_all   # 直接删除（不触发回调）
has_many :posts, dependent: :nullify      # 设置外键为NULL
has_many :posts, dependent: :restrict_with_error  # 有关联时阻止删除

# includes - 智能选择
Post.includes(:user)

# preload - 分离查询
Post.preload(:user)

# eager_load - JOIN查询
Post.eager_load(:user)

Post.includes(user: [:profile, :company])
Post.includes(:user, comments: :author)

# activerecord/lib/active_record/associations.rb
module ClassMethods
  def belongs_to(name, scope = nil, **options)
    reflection = Builder::BelongsTo.build(self, name, scope, options)
    Reflection.add_reflection(self, name, reflection)
  end
end

# activerecord/lib/active_record/associations/association.rb
class Association
  attr_reader :owner, :reflection

  def initialize(owner, reflection)
    @owner = owner
    @reflection = reflection
  end

  def load_target
    # 加载关联对象
  end
end