Question

This is almost answered in Difference Between find and Where with Relationships but not quite. (Notice how I subtly changed the question title!)
I do the 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 #<Libation:0x3b91e88>):

Nuts. I tried to find relevant docs, but found little. Or I am not looking in the right place.

Answer 1

#where returns an ActiveRecord::Relation object, which you can them perform additional methods on (such as #sum). However, #all executes the query returns an array of results, so when you do e.sum(...) you’re trying to perform #sum on an Array object rather than an ActiveRecord::Relation object.

You might try using #scoped instead:

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