ruby-on-rails – “where”和“find”之间的区别

这几乎在 Difference Between find and Where with Relationships回答,但并不完全. (请注意我是如何巧妙地改变问题标题的！)
我做查询

a = Libation.where("user_id = 1" )   # gets 10 records
b = a.sum("count * weight")          # Get right answer
c = Libation.where("user_id = 2" )   # gets 8 records
d = c.sum("count * weight")          # Get right answer

现在我做

e = Libation.all                # gets 18 records, good
f = e.sum("count * weight")     # BOOM! I get

NoMethodError (undefined method `+' for #<Libation:0x3b91e88>):

坚果.我试图找到相关的文档,但发现很少.或者我找不到合适的地方.

#where返回一个ActiveRecord :: Relation对象,您可以在其上执行其他方法(例如#sum).但是,#all执行查询返回结果数组,所以当你执行e.sum(…)时,你试图在Array对象而不是ActiveRecord :: Relation对象上执行#sum.

您可以尝试使用#scoped代替：

e = Libation.scoped
f = e.sum("count * weight")