def unwind(field)
      normalized = field.to_s
      name = aliases[normalized] || normalized
      push("$unwind" => name.__mongo_expression__)
    end
def unwind(field)
      normalized = field.to_s
      name = aliases[normalized] || normalized
      push("$unwind" => name.__mongo_expression__)
    end
Add the $unwind entry to the pipeline.

 @example Add the unwind.
   pipeline.unwind(:field)

 @param [ String, Symbol ] field The name of the field.

 @return [ Pipeline ] The pipeline.

 @since 2.0.0