Why is my JSON not transferred correctly from clientside to serverside

I have the following JSON:

{
  "fields":[
    {"name":"thom","techname":"rgom","description":"dfgkjd","type":"text"},
    {"name":"thom","techname":"rgom2","description":"dfgkjd","type":"text"}
   ]
}

When I post it to a NodeJS-server using this code:

    $.ajax({ 
      data : data, 
      type: 'POST',
      dataType: 'json',
      timeout: 10000,         
      url : '/schema/create', 
      success : function(response) {
        console.log(response)
      },
      complete : function() {
      },
      error: function(x, t, m) {
        if(t==="timeout") {
          alert("Timeout");
        } else {
          alert("Der opstod følgende fejl:" + t + x + m + ". Kontakt COWI");
        }
      }

    });

And insert it:

db.collection('schemas').insert(fields, {upsert:true}, function(err, result) {
  if(!err){
    console.log("written");
    console.log(result);

  }
});

In MongoDB I have:

"_id" : ObjectId("5512ed12ecacf6e01da7aaa4"),
"fields[0][name]" : "thom",
"fields[0][techname]" : "rgom",
"fields[0][description]" : "dfgkjd",
"fields[0][type]" : "text",
"fields[1][name]" : "thom",
"fields[1][techname]" : "rgom2",
"fields[1][description]" : "dfgkjd",
"fields[1][type]" : "text"

I was expecting:

{
  "_id":ObjectId("5512ed12ecacf6e01da7aaa4"),
  "fields":
  [
    {
      "name":"thom", 
      "techname" : "rgom",
      "description" : "dfgkjd",
      "type" : "text"
    },
    {
      "name":"thom", 
      "techname" : "rgom2",
      "description" : "dfgkjd",
      "type" : "text"
    }
  ]
}

EDIT: Logged before insert:

[ { 'fields[0][name]': 'thom',
    'fields[0][techname]': 'rgom',
    'fields[0][description]': 'dfgkjd',
    'fields[0][type]': 'text',
    'fields[1][name]': 'thom',
    'fields[1][techname]': 'rgom2',
    'fields[1][description]': 'dfgkjd',
    'fields[1][type]': 'text',
    _id: 5512ed12ecacf6e01da7aaa4 } ]

EDIT 2 Console logged before insert (using JSON.stringify()):

{
        "_id" : ObjectId("5512f0d4606391f41ddd16d1"),
        "{"fields":[{"name":"sdkljg","techname":"fgklj","description":"dfgklj","
type":"text"}]}" : ""
}