ruby-on-rails-3 – 如何在使用`average()’时在AREL中使用`to_sql`？

Review.where("reviewed_user_id = ?",self.reviewed_user_id).to_sql
#=> "SELECT `reviews`.* FROM `reviews` WHERE (reviewed_user_id = 3)"

Review.where("reviewed_user_id = ?",self.reviewed_user_id).average(:stars).to_sql
#=> undefined method `to_sql' for 3:Fixnum

原因是因为平均方法是ActiveRecord :: Relation,而不是Arel,它强制计算. @H_100_4@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]

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.

@H_100_4@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)"

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.

或者,您可以直接使用Arel,如下所示：

@H_100_4@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"