Am using the save option on mongoid on ruby on rails, its throwing me an error 

```
  can't convert Fixnum into String 

```

my model file are 

product.rb

```
 class Product

   include Mongoid::Document
   include Mongoid::Timestamps
   include Mongoid::Versioning

   field :product_role, :type => String
   field :status_id, type: Moped::BSON::ObjectId
   field :status_name,type: String
 end  

```

status.rb 

```
 class Status 

  include Mongoid::Document

  field :name, type: String

  index :name => 1

 end

```

controller code is 

```
        product_id = "523741002a6c6e8d1a000002",
        status_name = "Pending Review"
        product_detail_status = Product.where(id:product_id).first
        status = Status.where(name:status_name).first
        puts "##############"
        puts product_detail_status.id.to_s
        puts status.name.to_s
        puts status.id.to_s
        puts "##############"
        product_detail_status.status_name = status.name.to_s
        product_detail_status.status_id = status.id
        product_detail_status.save

```

i can able to view the values in puts, but its not saving by the way its throwing error on  

 product\_detail\_status.save as "can't convert Fixnum into String" 

```
 Completed 500 Internal Server Error in 657ms

TypeError - can't convert Fixnum into String:
mongoid (3.1.4) lib/mongoid/versioning.rb:53:in `revise'
activesupport (3.2.13) lib/active_support/callbacks.rb:407:in          `                
activesupport (3.2.13) lib/active_support/callbacks.rb:405:in `__run_callback'
activesupport (3.2.13) lib/active_support/callbacks.rb:385:in `_run_save_callbacks'
activesupport (3.2.13) lib/active_support/callbacks.rb:81:in `run_callbacks'
mongoid (3.1.4) lib/mongoid/callbacks.rb:130:in `run_callbacks'
mongoid (3.1.4) lib/mongoid/persistence/modification.rb:24:in `prepare'
mongoid (3.1.4) lib/mongoid/persistence/operations/update.rb:45:in `persist'
mongoid (3.1.4) lib/mongoid/persistence.rb:145:in `update'
mongoid (3.1.4) lib/mongoid/persistence.rb:82:in `save'

```

thanks in advance