我正在尝试从AREL获取sql,但是如果我使用average(：stars),它不起作用：
@H_301_2@这样做：
Review.where("reviewed_user_id = ?",self.reviewed_user_id).to_sql
#=> "SELECT `reviews`.* FROM `reviews` WHERE (reviewed_user_id = 3)"
@H_301_2@这会导致NoMethodError：
Review.where("reviewed_user_id = ?",self.reviewed_user_id).average(:stars).to_sql
#=> undefined method `to_sql' for 3:Fixnum
@H_301_2@所以这意味着to_sql正在调用AREL的结果,而不是AREL对象 – 但是为什么？
@H_301_2@如何获取生成的sql？
解决方法
原因是因为平均方法是ActiveRecord :: Relation,而不是Arel,它强制计算.
m = Review.where('id = ?',42).method(:average)
#=> #<Method: ActiveRecord::Relation(ActiveRecord::Calculations)#average>
m.source_location # or m.__file__ if you're on a different version of Ruby
#=> ["/Users/jtran/.rvm/gems/ruby-1.9.2-p0/gems/activerecord-3.0.4/lib/active_record/relation/calculations.rb",65]
@H_301_2@通过查看ActiveRecord :: Calculations的内部结构,您可以导出如何获取它使用的sql.
my_reviewed_user_id = 42
relation = Review.where('reviewed_user_id = ?',my_reviewed_user_id)
column = Arel::Attribute.new(Review.unscoped.table,:stars)
relation.select_values = [column.average]
relation.to_sql
#=> "SELECT AVG(\"reviews\".\"stars\") AS avg_id FROM \"reviews\" WHERE (reviewed_user_id = 42)"
@H_301_2@如果您正在控制台工作,请小心. ActiveRecord :: Relation缓存的东西,所以如果你一个一行地输入上面的控制台,它实际上不会工作,因为漂亮的打印强制的关系.然而,用分号分隔上述并没有新行将会奏效.
@H_301_2@或者,您可以直接使用Arel,如下所示：
my_reviewed_user_id = 42 reviews = Arel::Table.new(:reviews) reviews.where(reviews[:reviewed_user_id].eq(my_reviewed_user_id)).project(reviews[:stars].average).to_sql #=> "SELECT AVG(\"reviews\".\"stars\") AS avg_id FROM \"reviews\" WHERE \"users\".\"reviewed_user_id\" = 42"