Ruby-on-rails – the difference between “where” and “find”

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

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

NoMethodError (undefined method `+' for #):

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

This is almost in Difference Between f ind and Where with Relationships answer, but not complete. (Please note how I change the title of the question cleverly! )
I do a query

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 

Now I do

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

NoMethodError (undefined method `+' for #):

Nuts. I tried to find related documents, but found few .Or I can’t find a suitable place.

#where returns an ActiveRecord::Relation object on which you can perform other methods (e.g. #sum ). However, #all executes the query to return the result array, so when you execute e.sum(…), you try to execute #sum on the Array object instead of the ActiveRecord::Relation object.

You can try to use #scoped instead:

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