How to set Data type for Hash in rails (Mongoid)

Question

I have a mongoDB scheme something like this

new Schema({
 productDetail: [ {
                productYear:     String,
                productNumber:   String,
                productType:     [ { type: Schema.ObjectId, ref: 'type' } ]
              } ],
 created_at: string
            )}

Trying the same in Rails and mongoid I could not add child elements and data types to the Hash object

class Product
include Mongoid::Document
has_many :types
 field :productDetail, type: Hash
 field :created_at, type: string
end

is that even possible?