Problem:

The method BsonTemplateEvaluator::_evalElem only build a single level of template expansion, so it will expand the following correct

Input

{ _id : { "#VARIABLE" : "x" } }

Output

{ _id: 194 }

However at deeper levels, it does not expand correctly

Input

{ $set : { y :  { "#VARIABLE" : "x" } } }

Output

{ $set: { y: { #VARIABLE: "x" } } }