class Seller
  include Mongoid::Document
  field :seller_id, :type => Integer
  field :name, :type => String
  embeds_many :seller_products
  key :seller_id
end

class SellerProduct
  include Mongoid::Document
  field :product_id, :type => Integer
  key :product_id
  embedded_in :seller, :inverse_of => :seller_products
  validates_uniqueness_of :product_id
end

`

Validates_uniqueness_of doesnt work if i set my key to be :product_id

Uniqueness validator criteria becomes:
<Mongoid::Criteria
selector: {:product_id=>10, :_id=>{"$ne"=>"10"}},
options: {},
class: SellerProduct,
embedded: true>

When i remove the key it works

<Mongoid::Criteria

selector: {:product_id=>1, :_id=>{"$ne"=>BSON::ObjectId('4eaf0733a5d09fb9ef000006')}},
options: {},
class: SellerProduct,
embedded: true>