[Rails] sum decimals

I have a column "hours" which I want to sum up the decimals

Week.sum('hour')    => 22.5

But I want to "select by project_id" not the hole table.
Week.find_all_by_project_id(@project.id, :select => "workhour").sum  #undefined method `+' for #<Week hour: #<BigDecimal:7fd68b358400,'0.0',9(18)>>

I cant find anything ...some help would be great.

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