How to update array values with curl

In How to post array values via curl it gives a great example of how to set array values via curl:

$ curl -X POST http://localhost:3000/api/v1/shops.json -d \
  "shop[name]=Supermarket \
  &shop[products][]=fruit \
  &shop[products][]=eggs \
  &auth_token=a1b2c3d4"

What I'm wondering is if there's a way to update an existing array via curl without overwriting the values?

A little more info on what I'm trying to do - I'm trying to create a rails survey app with a HABTM relationship between the users/surveys. After a user takes the survey (in my Android app) it should update the join table with the user/survey so the user isn't served the same survey again. I've been trying to replicate this via curl but have been running into some trouble.

I'm able to update a survey title via this PUT request in curl:

curl -v -H 'Content-Type: application/json' -H 'Accept: application/json' -X PUT -d '{"title":"123dddTestingPlease rddfsfsfrdf","id":3,"users":[{"id":2,"email":"[email protected]","created_at":"2014-02-28T01:56:09.841Z","updated_at":"2014-02-28T01:56:09.879Z","admin":true,"auth_token":"7acac413cbc22049a3ebf074f45c9847"}]}' http://localhost:3000/surveys/1

But I'm having troubles updating the users array. Ideally it would just append the user I send to the existing user array.

Thanks in advance!

EDIT: First to answer your question - the problem is the user array doesn't seem to be updating at all when I do the curl request. What my end goal is to do is when the user completes the survey on the Android app, I want to send a PUT message to the rails server that will update that survey to contain that user (when the app checks to see if a survey is available, it checks to see if the user has taken the survey).

Here's my survey controller (I stripped everything but Update for readability):

class SurveysController < ApplicationController
  before_action :set_survey, only: [:show, :edit, :update, :destroy]

  # PATCH/PUT /surveys/1
  # PATCH/PUT /surveys/1.json
  def update
    respond_to do |format|
      if @survey.update(survey_params)
        format.html { redirect_to @survey, notice: 'Survey was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @survey.errors, status: :unprocessable_entity }
      end
    end
  end

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

    # Never trust parameters from the scary internet, only allow the white list through.
    def survey_params
      params.require(:survey).permit(:title, :users)
    end
end

I also tried a variant where I just send the user's auth_token and then I ID the user by that and try to add them to the users list, but that doesn't seem to work either. Here's what that code looked like:

def update
    @survey = Survey.find(params[:id])
    @survey.add_user params[:auth_token]
    redirect_to api_v1_survey_path(@survey)
 end

And the model:

class Survey < ActiveRecord::Base
  has_many :surveytizations
  has_many :questions, :through => :surveytizations
  has_many :users, :through => :completions
  has_many :completions

  def complete_survey (survey, user)
    survey.users << user
    survey.number_taken += 1
    if survey.number_taken == survey.survey_limit
      survey.survey_finished = true
    end
  end

  def add_user auth_token
    user = User.find_user_by_token auth_token
    completions.create(user_id: user.id)
  end

  def find_user(auth_token)
    user = User.find_user_by_token auth_token
    if (user != nil && completions.find_by(user_id: user.id) != nil) 
      return true
    else
      return false
    end
  end

  def self.check_survey (auth_token)
    Survey.all.each do |survey|
      next if survey.survey_finished
      next if survey.find_user auth_token
      return survey.id
    end
  end
end