def from_db(klass, attributes = nil, criteria = nil, selected_fields = nil)
      if criteria
        selected_fields ||= criteria.options[:fields]
      end
      type = (attributes || {})[TYPE]
      if type.blank?
        obj = klass.instantiate(attributes, selected_fields)
        if criteria && criteria.association && criteria.parent_document
          obj.set_relation(criteria.association.inverse, criteria.parent_document)
        end
        obj
      else
        camelized = type.camelize

        # Check if the class exists
        begin
          constantized = camelized.constantize
        rescue NameError
          raise Errors::UnknownModel.new(camelized, type)
        end

        # Check if the class is a Document class
        if !constantized.respond_to?(:instantiate)
          raise Errors::UnknownModel.new(camelized, type)
        end

        constantized.instantiate(attributes, selected_fields)
      end
    end
def from_db(klass, attributes = nil, criteria = nil, selected_fields = nil)
      if criteria
        selected_fields ||= criteria.options[:fields]
      end
      type = (attributes || {})[TYPE]
      if type.blank?
        obj = klass.instantiate(attributes, selected_fields)
        if criteria && criteria.association && criteria.parent_document
          obj.set_relation(criteria.association.inverse, criteria.parent_document)
        end
        obj
      else
        camelized = type.camelize

        # Check if the class exists
        begin
          constantized = camelized.constantize
        rescue NameError
          raise Errors::UnknownModel.new(camelized, type)
        end

        # Check if the class is a Document class
        if !constantized.respond_to?(:instantiate)
          raise Errors::UnknownModel.new(camelized, type)
        end

        constantized.instantiate(attributes, selected_fields)
      end
    end
Builds a new +Document+ from the supplied attributes loaded from the
 database.

 If a criteria object is given, it is used in two ways:
 1. If the criteria has a list of fields specified via #only,
    only those fields are populated in the returned document.
 2. If the criteria has a referencing association (i.e., this document
    is being instantiated as an association of another document),
    the other document is also populated in the returned document's
    reverse association, if one exists.

 @example Build the document.
   Mongoid::Factory.from_db(Person, { "name" => "Durran" })

 @param [ Class ] klass The class to instantiate from if _type is not present.
 @param [ Hash ] attributes The document attributes.
 @param [ Criteria ] criteria Optional criteria object.
 @param [ Hash ] selected_fields Fields which were retrieved via
   #only. If selected_fields are specified, fields not listed in it
   will not be accessible in the returned document.

 @return [ Document ] The instantiated document.