获取 has_many 和 belongs_to 生成的 SQL

有的时候我们想要分析 Rails ORM 生成的 SQL has_many 关系 比较容易获取 belongs_to 关系 稍微麻烦一些 关键点在于 ActiveRecord::Relation#to_sql

https://github.com/rails/rails/blob/v3.2.17/activerecord/lib/active_record/relation.rb#L458

class Customter
  has_many :orders
end

class Order
  belongs_to :customter
end

Customter.first.orders.to_sql
# => SELECT `orders`.* FROM `orders`  WHERE `orders`.`customer_id` = 136
# => BTW 136 is customer_id

Order.first.association(:customer).association_scope.to_sql
# => SELECT `customers`.* FROM `customers`  WHERE `customers`.`id` = 136
# => BTW 136 is customer_id

Customter.first.orders.proxy_association.association_scope.class
# => ActiveRecord::Relation

Order.first.association(:customer).association_scope.class
# => ActiveRecord::Relation

关于更多技术实现细节，可以研究 ActiveRecord::Associations#association

https://github.com/rails/rails/blob/v3.2.17/activerecord/lib/active_record/associations.rb#L155