I got a Completed 500 Internal Server Error when I saved a duplicated record to a table which has validate of uniqueness and a composite index

gistfile1.txt

User can like products, but as one product it should only be liked by one user once. So I add a composite index of product_id and user_id on table like and add a validate on model like.

Here is the codes:

like.rb
class Like < ActiveRecord::Base
  belongs_to :user
  belongs_to :product
  accepts_nested_attributes_for :product
  validates :user_id, uniqueness: {
    scope: :product_id
  }
end

likes_controller.rb
class LikesController < ApplicationController
  def create
    @like = Like.new user_id: current_user.id,
                  product_id: like_params[:product_id]
    @like.product.increment :likenumber

    respond_to do |format|
      if @like.save
        format.json { render json: @like, status: :created }
      else
        format.json { render json: @like.errors, status: :nprocessable_entity }
      end
    end
  end

  def destroy
  end

  private
    def like_params
      params.permit(:product_id)
    end
end

show.html.erb
<%= link_to raw("<i class=\"fa fa-heart-o fa-2x\"></i><i class=\"fa fa-heart fa-2x\"></i><br><span class=\"label\">#{@product.likenumber}</span>"), [@product, @like], remote: true, method: :post, :class => "slidebutton like" %>

Here is the server log:
Started POST "/products/4/likes" for 10.0.1.14 at 2015-09-09 09:53:09 +0800
Processing by LikesController#create as JS
  Parameters: {"product_id"=>"4"}
  User Load (0.4ms)  SELECT  "users".* FROM "users"  WHERE "users"."id" = 1  ORDER BY "users"."id" ASC LIMIT 1
  Product Load (0.1ms)  SELECT  "products".* FROM "products"  WHERE "products"."id" = $1 LIMIT 1  [["id", 4]]
   (0.1ms)  BEGIN
  Like Exists (0.2ms)  SELECT  1 AS one FROM "likes"  WHERE ("likes"."user_id" = 1 AND "likes"."product_id" = 4) LIMIT 1
   (0.1ms)  ROLLBACK
Completed 500 Internal Server Error in 9ms (Views: 0.3ms | ActiveRecord: 0.9ms)

And the real response:
{"user_id":["Has been taken."]}