Summing Mongoid BigDecimal - ruby-on-rails-3

Question

Mongoid 3 documentation shows you can do simple sums using something like this: Band.sum(:likes)
I have the following simple models:
class Project
...
has_many :subprojects
...
end
class Subproject
...
field :subtotal, :type => BigDecimal, :default => 0
...
end
How do I sum subtotal across each Project?
For example, I tried
Project.first.subprojects.sum(:subtotal) it returns 0.
But
Project.first.subprojects.first.subtotal returns #<BigDecimal:7fcb0d77b958,'0.11054E3',18(18)>
Any suggestions?

Answer 1

I tested how Mongoid stores BigDecimal:
rails c:
class Test
include Mongoid::Document
field :decimal, type: BigDecimal
end
test = Test.new
test.decimal = BigDecimal.new(123, 456)
test.save
I then queryed the database:
> db.tests.find()
{ "_id" : ObjectId("52e0ed4bb2e8c9ea34000001"), "decimal" : "123.0" }
You clearly see that the BigDecimal is stored as a string, thus explaning why you can't sum it.
Howevery, you can try to parse it then sum it using a map/reduce :
map = %Q{
function() {
emit("BigDecimalSum", { decimal: parseFloat(this.decimal) });
}
}
reduce = %Q{
function(key, values) {
var result = { sum: 0 };
values.forEach(function(value) {
result.sum += value.decimal;
});
return result;
}
}
Test.map_reduce(map, reduce).out(inline: true)
I've just tester the parseFloat in mongo client and it works, so this map/reduce should work too.

Answer 2

To sum BigDecimal fields in Mongoid you use the block form of #sum:
sum = 0
Project.first.subprojects.sum do |subproject|
sum += subproject.subtotal
end