Was trying to query products based on id passed in, but I got all of the records instead of the one with category id given

index.js

var Product = new Schema({
    quantity: { type: Number }
  , title: { type: String, trim: true }
  , category: { type: ObjectId, ref: 'Category' }
})

var Model = db.model('Product', Product);

Product
  .statics
  .byCategoryId = function(id, next) {
    var q = Model.find()

    q.where('category', id) // << Works!
    // q.where({ category: id }) // << Nope, why?
    q.exec(next);
  }