cupakromer/gist:3879718

## gistfile1.md

      
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              gistfile1.md
            
          
    class MyModel < ActiveRecord::Base
  has_many :things
end

model = MyModel.new
model.things.build do
  #stuff...
end

model.things.sum(&:method)
#=> TypeError: nil can't be coerced into BigDecimal

model.things.class
#=> Array

model.things.to_a.sum(&:method)
#=> #<BigDecimal:7ff4d34ca7c8,'0.0',9(36)>

model.things.map(&:method).sum
#=> #<BigDecimal:7ff4d3373b40,'0.0',9(36)>
It seems in (Rails 3.2.8) that has_many is creating collection.sum.
According to the code, it will always try to hit the database using a SQL sum.
However, in this particular instance, the model isn't saved. So there's nothing in
the database. I would think it should pull from the cached copy.
Also, there is the potential where the model is saved, but the collection was
modified but not saved yet (i.e. with a build); this could cause un-expected
sums.