MongoDB relationship

Hello,

I am using the "MongoDB Writer node" and Knime generate special value.

MongoDB browse this type of JSON value :

{ 
	"_id" : ObjectId("583d1a71fed417940b00002c"), 
	"link" : "http://www.google.ch", 
	"country" : "Suisse", 
	"category" : DBRef("category", ObjectId("583d1a1ffed417c00a00002a"), "dbname") }

And Knime browse this type of JSON value :

{
  "_id" : {
    "$oid" : "583d1a71fed417940b00002c"
  },
  "link" : "http://www.google.ch",
  "country" : "Suisse",
  "category" : {
    "$ref" : "category",
    "$id" : {
      "$oid" : "583d1a1ffed417c00a00002a"
    },
    "$db" : "dbname"
  }
}

So, $oid matches ObjectId, $db matches DBRef, ...

Why is it not similar json format ?

Where can I find translation between mongodb and Knime ?

Witch query do I have to give to Knime to make relationship for category ? What kind of json file do I have to give to Knime for relationship (category for a link) ?

Do you have documentation ?

Thanks for helping,

Thierry