(* I am raising this ticket for driver, but the solution may also partly be in Mongoid)

While testing for MONGOID-4500, I observed the following behavior:
 

class Band
   ...
   field :mojo, type: Object
end

band = Band.create!(mojo: BigDecimal('123'))

band.mojo #=> returns BSON::Decimal128 object !!

This is problematic to get a BSON::Decimal128, because: * BSON::Decimal128#numeric? returns false

• BSON::Decimal128 does not implement Ruby stdlib Numeric interface, i.e. methods like #to_i, #to_f, #coerce etc.

I think Mongo team should decide whether to:

• A) Make BSON::Decimal128 should be a proper Numeric-like object, i.e. implement #to_i, #to_f, #coerce and .is_a?(Numeric), numeric? etc. These could delegate to to_big_decimal for now. This should then be done for all BSON numeric types, e.g. BSON::Int32, BSON::Int64, etc.
• B) Always convert BSON::Decimal128 to BigDecimal ideally at the driver level--never expose it to the user in a way that it could accidentally get mixed up in numeric calculations. Note that this is the current approach I believe with Integer, Float, etc.
• C) Do both A and B – have (B) "always convert to BigDecimal in driver" be the golden path, but implement (A) for safety to avoid app errors.

Approach B or C would be my preference.