Ruby on Rails ActiveResource not saving resource model properly

ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-linux],

Rails 4.2.5

I have two projects. from 1st project i am getting data into second project through api.

User model in 1st project:

class User < ActiveRecord::Base
 has_many :cars
end

Car model in 1st project:

class Car < ActiveRecord::Base
  belongs_to :user
end

Car model(remote) in 2nd project:

class Car < ActiveResource::Base
   self.site  = 'https://myeasyb-vssram.c9users.io'
   self.format = :json
end

Gpstablecontroller(2nd project):

  class GpstablesController < ApplicationController
  before_action :set_gpstable, only: [:show, :edit, :update, :destroy]

  # GET /gpstables
  # GET /gpstables.json
  def index
    @gpstables = Gpstable.all
  end

  # GET /gpstables/1
  # GET /gpstables/1.json
  def show
  end

  # GET /gpstables/new
  def new
    @gpstable = Gpstable.new
    @gpstables = Gpstable.all
  end

  # GET /gpstables/1/edit
  def edit
     @gpstables = Gpstable.all
  end

  # POST /gpstables
  # POST /gpstables.json
  def create
    @cars = Car.all
    @gpstable = Gpstable.new(gpstable_params)
     @cars.each do |car|
      if @gpstable.car_id == car.id
        @car = car 
      end
    end

     @car.update_attribute(:gpss,  @gpstable.device_id)

    respond_to do |format|
      if @gpstable.save
        format.html { redirect_to gpstables_url, notice: 'Gpstable was successfully created.' }
        format.json { render :show, status: :created, location: @gpstable }
      else
        format.html { render :new }
        format.json { render json: @gpstable.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /gpstables/1
  # PATCH/PUT /gpstables/1.json
  def update
    respond_to do |format|
      if @gpstable.update(gpstable_params)
        Car.all.each do |car|
          if @gpstable.car_id == car.id.to_json
            @car = car
          end

          if @gpstable.device_id == car.gpss
            car.gpss = 0
            car.save!
          end

         end
        @car.gpss = @gpstable.device_id

        @car.save!
        format.html { redirect_to @gpstable, notice: 'Gpstable was successfully updated.' }
        format.json { render :show, status: :ok, location: @gpstable }
      else
        format.html { render :edit }
        format.json { render json: @gpstable.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /gpstables/1
  # DELETE /gpstables/1.json
  def destroy

     @cars.each do |car|
     if @gpstable.device_id == car.gpss
            car.gpss = 0
            car.user_id = @gpstable.user_id
            car.save
     end
     end
     @gpstable.destroy

    respond_to do |format|
      format.html { redirect_to gpstables_url, notice: 'Gpstable was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_gpstable
      @gpstable = Gpstable.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def gpstable_params
      params.require(:gpstable).permit(:device_id, :car_id, :user_id)
    end
end

when creating gpstable record i want to update Gpss attribute of car model(remote, calling through api).

it is updating gpss attribute.But it is changing all foriegnkeys including user_id attribute of car model to null.

using devise for users in 1st project.

The problem is i am giving user_id to current user_id in car_params. so i was unable to edit this through resource model. so i changed this to create action. In my first app i have car controller:

  def car_params
      params.require(:car).permit(:name, :model, :colour, :ac, :gpss, :wifi, :luggage, :cfare, :card, :crfare, :no, :nos, :user_id, {carpicss: []} ).**merge(user: :current_user)**
    end

i removed .merge(user: :current_user) from above code.

and added this in create action

  def create
     @car = Car.new(car_params)
     @car.card=" #{@car.name} : #{@car.no}"
     @car.user_id=current_user.id   #added here to save current user_id 
    respond_to do |format|
      if @car.save
        format.html { redirect_to cars_path, notice: 'Car was successfully created.' }
        format.json { render :show, status: :created, location: cars_path }
      else
        format.html { render :new }
        format.json { render json: @car.errors, status: :unprocessable_entity }
      end
      @car.reload
    end
  end