Hi, 大家好！

繼昨天的研究，我們目前知道CollectionProxy 涉及到三個objects：owner、target、Reflection

以上篇的例子來看：

irb(main):006:0> User.first.posts.class
  User Load (3.2ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1]]
Post::ActiveRecord_Associations_CollectionProxy < ActiveRecord::Associations::CollectionProxy

owner 為User.first、target 為posts，那Reflection object 呢？

我們先來看看在Rails API 搜尋Reflection 可以得到的結果：

從圖裡可看到Reflection 是ActiveRecord 下的一個類別：

Reflection enables the ability to examine the associations and aggregations of Active Record classes and objects. This information, for example, can be used in a form builder that takes an Active Record object and creates input fields for all of the attributes depending on their type and displays the associations to other objects.

**翻譯蒟蒻：Reflection 具有可檢視associations/aggregations 裡物件的能力。而檢視到的資訊可透過form builder 來建立目標的attributes。

猜測這應該解釋了我們為什麼可以這麼建立資料吧：

irb(main):017:0> User.first.posts.create(title: "IThome", body: "IThome rocks")
  User Load (0.8ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1]]
   (0.2ms)  BEGIN
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
  Post Create (8.0ms)  INSERT INTO "posts" ("userId", "title", "body", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["userId", 1], ["title", "IThome"], ["body", "IThome rocks"], ["created_at", "2022-10-01 13:20:49.585730"], ["updated_at", "2022-10-01 13:20:49.585730"]]
   (2.6ms)  COMMIT
#<Post:0x00007fb4a8338a38> {
            :id => 101,
        :userId => 1,
         :title => "IThome",
          :body => "IThome rocks",
    :created_at => Sat, 01 Oct 2022 13:20:49 UTC +00:00,
    :updated_at => Sat, 01 Oct 2022 13:20:49 UTC +00:00
}

Reflectin Class的第二句說明：

MacroReflection class has info for AggregateReflection and AssociationReflection classes.

**翻譯蒟蒻：MacroReflection 有associations/aggregations 的資訊

可看到這邊還是有點不懂，那MacroReflection 是什麼呢：

ActiveRecord::Reflection::MacroReflection < ActiveRecord::Reflection::AbstractReflection
activerecord/lib/active_record/reflection.rb

Base class for AggregateReflection and AssociationReflection. Objects of AggregateReflection and AssociationReflection are returned by the Reflection::ClassMethods.

**翻譯蒟蒻：結合了這兩個類別的說明，應該是指Reflection 下有個MacroReflection，當使用Reflection 的類別方法時會回傳MacroReflection，上面有associations/aggregations 的資訊

而先看看ActiveRecord::Reflection::MacroReflection 的方法：

大概能理解我們可以這樣做：

ActiveRecord::Reflection::MacroReflection.new(:user, { id: 1 }, ...... }

# 好吧，後面我還不知道怎麼做 XD 但我們能從下方的實體方法來一窺一二

irb(main):008:0> User.reflect_on_association(:posts)
#<ActiveRecord::Reflection::HasManyReflection:0x00007fb4a32dfca8 @name=:posts, @scope=nil, @options={:foreign_key=>:userId}, @active_record=User(id: integer, created_at: datetime, updated_at: datetime, name: string, username: string, email: text, phone: text, website: text), @klass=Post(id: integer, userId: integer, title: text, body: text, created_at: datetime, updated_at: datetime), @plural_name="posts", @type=nil, @foreign_type=nil, @constructable=true, @inverse_name=nil, @class_name="Post", @foreign_key=:userId, @active_record_primary_key="id", @inverse_which_updates_counter_cache=nil>

讓我們先跳去看看Reflection 的類別方法：

.macro -> 可得知彼此（User vs Post）的關聯方法

irb(main):020:0> User.reflect_on_association(:posts).macro
:has_many

.reflect_on_all_associations -> 回傳User 和其他Model 的Reflection

irb(main):021:0> User.reflect_on_all_associations
[
    [0] #<ActiveRecord::Reflection::HasManyReflection:0x00007fb4a32dfca8 @name=:posts, @scope=nil, @options={:foreign_key=>:userId}, @active_record=User(id: integer, created_at: datetime, updated_at: datetime, name: string, username: string, email: text, phone: text, website: text), @klass=Post(id: integer, userId: integer, title: text, body: text, created_at: datetime, updated_at: datetime), @plural_name="posts", @type=nil, @foreign_type=nil, @constructable=true, @inverse_name=nil, @class_name="Post", @foreign_key=:userId, @active_record_primary_key="id", @inverse_which_updates_counter_cache=nil>,
    [1] #<ActiveRecord::Reflection::HasManyReflection:0x00007fb4a8669220 @name=:todos, @scope=nil, @options={:foreign_key=>:userId}, @active_record=User(id: integer, created_at: datetime, updated_at: datetime, name: string, username: string, email: text, phone: text, website: text), @klass=nil, @plural_name="todos", @type=nil, @foreign_type=nil, @constructable=true>,
    [2] #<ActiveRecord::Reflection::HasManyReflection:0x00007fb4a86594d8 @name=:albums, @scope=nil, @options={:foreign_key=>:userId}, @active_record=User(id: integer, created_at: datetime, updated_at: datetime, name: string, username: string, email: text, phone: text, website: text), @klass=nil, @plural_name="albums", @type=nil, @foreign_type=nil, @constructable=true>
]

# 若加上(macro = nil) 在後方當引數，就能指定要查has_one, has_many, belongs_to 的所有Reflection，如：
irb(main):023:0> User.reflect_on_all_associations(:has_many)
# 結果跟上面都一樣，略

研究到此，大概能釐清頭緒，得知CollectionProxy 涉及到的三個objects：owner、target、Reflection，分別是：

• owner: User.first
• target: posts
• Reflection: 兩者關聯的資訊

而CollectionProxy 就是綜括以上的結果集啦~~

今天的研究就到這邊，謝謝~~

明天暫定繼續研究CollectionProxy 的方法，和稍微探討一下繼承自Relation 的東東：

ActiveRecord::Associations::CollectionProxy < Relation