自動加載機制

require "active_support"
require "active_support/rails"
require "active_model"
require "arel"
require "yaml"

require "active_record/version"
require "active_model/attribute_set"

module ActiveRecord
  extend ActiveSupport::Autoload

  autoload :Base
  autoload :Callbacks
  autoload :Core
  autoload :ConnectionHandling
  autoload :CounterCache
  autoload :DynamicMatchers
  autoload :Enum
  autoload :InternalMetadata
  autoload :Explain
  autoload :Inheritance
  autoload :Integration
  autoload :Migration
  autoload :Migrator, "active_record/migration"
  autoload :ModelSchema
  autoload :NestedAttributes
  autoload :NoTouching
  autoload :TouchLater
  autoload :Persistence
  autoload :QueryCache
  autoload :Querying
  autoload :CollectionCacheKey
  autoload :ReadonlyAttributes
  autoload :RecordInvalid, "active_record/validations"
  autoload :Reflection
  autoload :RuntimeRegistry
  autoload :Sanitization
  autoload :Schema
  autoload :SchemaDumper
  autoload :SchemaMigration
  autoload :Scoping
  autoload :Serialization
  autoload :StatementCache
  autoload :Store
  autoload :Suppressor
  autoload :Timestamp
  autoload :Transactions
  autoload :Translation
  autoload :Validations
  autoload :SecureToken

  eager_autoload do
    autoload :ActiveRecordError, "active_record/errors"
    autoload :ConnectionNotEstablished, "active_record/errors"
    autoload :ConnectionAdapters, "active_record/connection_adapters/abstract_adapter"

    autoload :Aggregations
    autoload :Associations
    autoload :AttributeAssignment
    autoload :AttributeMethods
    autoload :AutosaveAssociation

    autoload :LegacyYamlAdapter

    autoload :Relation
    autoload :AssociationRelation
    autoload :NullRelation

    autoload_under "relation" do
      autoload :QueryMethods
      autoload :FinderMethods
      autoload :Calculations
      autoload :PredicateBuilder
      autoload :SpawnMethods
      autoload :Batches
      autoload :Delegation
    end

    autoload :Result
    autoload :TableMetadata
    autoload :Type
  end

  module Coders
    autoload :YAMLColumn, "active_record/coders/yaml_column"
    autoload :JSON, "active_record/coders/json"
  end

  module AttributeMethods
    extend ActiveSupport::Autoload

    eager_autoload do
      autoload :BeforeTypeCast
      autoload :Dirty
      autoload :PrimaryKey
      autoload :Query
      autoload :Read
      autoload :TimeZoneConversion
      autoload :Write
      autoload :Serialization
    end
  end

  module Locking
    extend ActiveSupport::Autoload

    eager_autoload do
      autoload :Optimistic
      autoload :Pessimistic
    end
  end

  module ConnectionAdapters
    extend ActiveSupport::Autoload

    eager_autoload do
      autoload :AbstractAdapter
    end
  end

  module Scoping
    extend ActiveSupport::Autoload

    eager_autoload do
      autoload :Named
      autoload :Default
    end
  end

  module Tasks
    extend ActiveSupport::Autoload

    autoload :DatabaseTasks
    autoload :SQLiteDatabaseTasks, "active_record/tasks/sqlite_database_tasks"
    autoload :MySQLDatabaseTasks,  "active_record/tasks/mysql_database_tasks"
    autoload :PostgreSQLDatabaseTasks,
      "active_record/tasks/postgresql_database_tasks"
  end

  autoload :TestFixtures, "active_record/fixtures"

  def self.eager_load!
    super
    ActiveRecord::Locking.eager_load!
    ActiveRecord::Scoping.eager_load!
    ActiveRecord::Associations.eager_load!
    ActiveRecord::AttributeMethods.eager_load!
    ActiveRecord::ConnectionAdapters.eager_load!
  end
end

ActiveSupport.on_load(:active_record) do
  Arel::Table.engine = self
end

ActiveSupport.on_load(:i18n) do
  I18n.load_path << File.expand_path("active_record/locale/en.yml", __dir__)
end

YAML.load_tags["!ruby/object:ActiveRecord::AttributeSet"] = "ActiveModel::AttributeSet"
YAML.load_tags["!ruby/object:ActiveRecord::Attribute::FromDatabase"] = "ActiveModel::Attribute::FromDatabase"
YAML.load_tags["!ruby/object:ActiveRecord::LazyAttributeHash"] = "ActiveModel::LazyAttributeHash"

Active Record是Rails的ORM功能實現。上面代碼使用了ActiveSupport::Autoload模塊，該模塊定義了autoload方法。代碼首次引用模塊時，這個方法通過命名約定自動識別和加載該模塊。
我們先看下Autoload模塊代碼的具體實現：

# frozen_string_literal: true

require "active_support/inflector/methods"

module ActiveSupport
  # Autoload and eager load conveniences for your library.
  #
  # This module allows you to define autoloads based on
  # Rails conventions (i.e. no need to define the path
  # it is automatically guessed based on the filename)
  # and also define a set of constants that needs to be
  # eager loaded:
  #
  #   module MyLib
  #     extend ActiveSupport::Autoload
  #
  #     autoload :Model
  #
  #     eager_autoload do
  #       autoload :Cache
  #     end
  #   end
  #
  # Then your library can be eager loaded by simply calling:
  #
  #   MyLib.eager_load!
  module Autoload
    def self.extended(base) # :nodoc:
      base.class_eval do
        @_autoloads = {}
        @_under_path = nil
        @_at_path = nil
        @_eager_autoload = false
      end
    end

    def autoload(const_name, path = @_at_path)
      unless path
        full = [name, @_under_path, const_name.to_s].compact.join("::")
        path = Inflector.underscore(full)
      end

      if @_eager_autoload
        @_autoloads[const_name] = path
      end

      super const_name, path
    end

    def autoload_under(path)
      @_under_path, old_path = path, @_under_path
      yield
    ensure
      @_under_path = old_path
    end

    def autoload_at(path)
      @_at_path, old_path = path, @_at_path
      yield
    ensure
      @_at_path = old_path
    end

    def eager_autoload
      old_eager, @_eager_autoload = @_eager_autoload, true
      yield
    ensure
      @_eager_autoload = old_eager
    end

    def eager_load!
      @_autoloads.each_value { |file| require file }
    end

    def autoloads
      @_autoloads
    end
  end
end

首先看到autoload方法，如果再沒有傳path參數的情況下，autoload會根據當前的類（模塊）名，和傳入的const_name,組成一個path，最后根據傳入的const_name,和path，調用ruby原生的autoload加載模塊。如果通過傳遞block的方式調用eager_autoload,這種情況下調用autoload，會把相應的模塊加入到@_eager_autoload中，然后可以通過eager_load!方法直接require對應path下的文件。以及相應的autoload_at和autoload_under，也可以通過block的方式傳遞path，改變默認的@_at_path,執(zhí)行autoload，真的是非常巧妙的方式。
ruby原生的autoload部分，參考：http://www.itdecent.cn/p/d9dcbed59a82

Validations模塊

ActiveRecord::Base包含了 ActiveRecord::Validations模塊，里面卻找不到我們要用的validate方法。這個模塊來自Active Model，Active Record的一個依賴庫。為什么作者要把validate方法定義在別的庫呢，其實早期的rails沒有Active Model庫，那時候validate方法定義在Active Record里，但是隨著Active Record的不斷壯大，開發(fā)者發(fā)現其實這是兩項獨立的工作，一項與數據庫操作相關，比如保存和加載數據庫。但另一項是處理對象模型的，比如維護對象屬性，或者跟蹤對象屬性的有效性。于是就被分成了兩個庫。