Mongoose: Using $in operator with a JSON object

I am newbie to Nodejs and mongodb.

I was trying to get data for the following scenario. Consider 3 schemas as follows Profile schema

var ProfileSchema = new Schema({
username: {type: String, match: /^[a-zA-Z0-9_.-]+$/, unique: true},
name: String});

Posts schema

var PostsSchema = new Schema({
profileid: {type: ObjectId, ref: 'Profile'},
message: {type: String, match: /^.{1,160}$/} });

Follows schema

var FollowSchema = new Schema({
profileid: {type: ObjectId, ref: 'Profile'},
followingid: {type: ObjectId, ref: 'Profile'}});

Which is similar to the twitter hierarchy.

To get all the posts from my followers I tried the following

Follows.find({'profileid' : '500d18823e792d8814000001'}).select('followingid -_id').limit(20).sort({addedon: 'desc'}).execFind(function (arr,followings) {
    Posts.find({profileid: {$in: followings}}).limit(20).execFind(function (arr,data) {
        console.log(data);
    });
});

But it is not working. Please guide the best way to get this.

Thanks for your support