Nested object creation with JSON in Rails

Question

How do I pass JSON to a RAILS application so it will created nested child objects in a has_many relationship?

Here's what I have so far:

Two model objects.

class Commute < ActiveRecord::Base
  has_many :locations
  accepts_nested_attributes_for :locations, :allow_destroy => true
end

class Location < ActiveRecord::Base
  belongs_to :commute
end

With Commute, I have a standard controller set up. I'd like to be able to create a Commute object as well as several child Location objects in a single REST call using JSON. I've been trying things like this:

curl -H "Content-Type:application/json" -H "Accept:application/json" 
-d "{\"commute\":{\"minutes\":0, 
\"startTime\":\"Wed May 06 22:14:12 EDT 2009\", 
\"locations\":[{\"latitude\":\"40.4220061\",
\"longitude\":\"40.4220061\"}]}}"  http://localhost:3000/commutes

Or more readable, the JSON is:

{
    "commute": {
        "minutes": 0,
        "startTime": "Wed May 06 22:14:12 EDT 2009",
        "locations": [
            {
                "latitude": "40.4220061",
                "longitude": "40.4220061"
            }
        ]
    }
}

When I execute that, I get this output:

Processing CommutesController#create (for 127.0.0.1 at 2009-05-10 09:48:04) [POST]
  Parameters: {"commute"=>{"minutes"=>0, "locations"=>[{"latitude"=>"40.4220061", "longitude"=>"40.4220061"}], "startTime"=>"Wed May 06 22:14:12 EDT 2009"}}

ActiveRecord::AssociationTypeMismatch (Location(#19300550) expected, got HashWithIndifferentAccess(#2654720)):
  app/controllers/commutes_controller.rb:46:in `new'
  app/controllers/commutes_controller.rb:46:in `create'

It looks like the locations JSON array is being read in, but not interpreted as a Location object.

I can easily change either the client or the server, so the solution could come from either side.

So, does RAILS make this easy for me to do? Or do I need to add in some support for this to my Commute object? Perhaps add a from_json method?

Thanks for any help.

Answer 1

As I've been working through this, one solution that works is to modify my controller. But this doesn't seem the "rails" way of doing it, so please still let me know if there's a better way.

def create
    locations = params[:commute].delete("locations");
    @commute = Commute.new(params[:commute])

    result = @commute.save

    if locations 
      locations.each do |location|
        @commute.locations.create(location)
      end
    end


    respond_to do |format|
      if result
        flash[:notice] = 'Commute was successfully created.'
        format.html { redirect_to(@commute) }
        format.xml  { render :xml => @commute, :status => :created, :location => @commute }
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @commute.errors, :status => :unprocessable_entity }
      end
    end
  end

Answer 2

May I ask why you are using JSON to create new objects in your Rails app? Typically you could use a POST HTTP request and send the parameters of your new object in the POST request's body. I've never heard of using JSON to send data back to Rails.

Answer 3

Figured it out, the locations object should have been called locations_attributes to match up with the Rails nested object creation naming scheme. After doing that, it works perfectly with a default Rails controller.

{
    "commute": {
        "minutes": 0,
        "startTime": "Wed May 06 22:14:12 EDT 2009",
        "locations_attributes": [
            {
                "latitude": "40.4220061",
                "longitude": "40.4220061"
            }
        ]
    }
}