Find all _id where attribute is object

is_object.js

//http://stackoverflow.com/questions/29747782/in-mongodb-how-to-find-documents-where-property-is-object-but-not-array
//Find all entries where gyms attribute is an object

var arr = [];
db.test.find().forEach(
  function isObj(doc) {
    if( typeof doc.gyms === 'object') {
      arr.push(doc._id);
    }
  }
}

print( arr.join("\r\n") );