design documents issue in pouchdb

design_document.js

function(doc) {
  if ( doc.collection === "foobar") {
      for(var i = 0; i < doc.belongsTo.length; ++i) {
          var belongTo = doc.belongsTo[i];
          if ( !!belongTo.length) {
              emit(belongTo);
          }
      }
  }
}

document.json

{
  "_id" : "XXXX-XXXX-XXXX-XXXX",
  "title" : "....",
  "belongsTo" : [
    "item1",
    "item2",
    "item3"
  ],
  "collection" : "foobar"
}