accepts_nested_attributes_for with belongs_to polymorphic

I would like set up a polymorphic relation with accepts_nested_attributes_for. Here is the code:

class Contact <ActiveRecord::Base
  has_many :jobs, :as=>:client
end

class Job <ActiveRecord::Base
  belongs_to :client, :polymorphic=>:true
  accepts_nested_attributes_for :client
end

When I try to access Job.create(..., :client_attributes=>{...} gives me NameError: uninitialized constant Job::Client

I've also had a problem with the "ArgumentError: Cannot build association model_name. Are you trying to build a polymorphic one-to-one association?"

And I found a better solution for this kind of problem. You can use native method. Lets look to the nested_attributes implementation, inside Rails3:

elsif !reject_new_record?(association_name, attributes)
  method = "build_#{association_name}"
  if respond_to?(method)
    send(method, attributes.except(*UNASSIGNABLE_KEYS))
  else
    raise ArgumentError, "Cannot build association #{association_name}. Are you trying to build a polymorphic one-to-one association?"
  end
end

So actually what do we need to do here? Is just to create build_#{association_name} inside our model. I've did totally working example at the bottom:

class Job <ActiveRecord::Base
  CLIENT_TYPES = %w(Contact)

  attr_accessible :client_type, :client_attributes

  belongs_to :client, :polymorphic => :true

  accepts_nested_attributes_for :client

  protected

  def build_client(params, assignment_options)
    raise "Unknown client_type: #{client_type}" unless CLIENT_TYPES.include?(client_type)
    self.client = client_type.constantize.new(params)
  end
end

I finally got this to work with Rails 4.x. This is based off of Dmitry/ScotterC's answer, so +1 to them.

STEP 1. To begin, here is the full model with polymorphic association:

# app/models/polymorph.rb
class Polymorph < ActiveRecord::Base
  belongs_to :associable, polymorphic: true

  accepts_nested_attributes_for :associable

  def build_associable(params)
    self.associable = associable_type.constantize.new(params)
  end
end

# For the sake of example:
# app/models/chicken.rb
class Chicken < ActiveRecord::Base
  has_many: :polymorphs, as: :associable
end

Yes, that's nothing really new. However you might wonder, where does polymorph_type come from and how is its value set? It's part of the underlying database record since polymorphic associations add <association_name>_id and <association_name>_type columns to the table. As it stands, when build_associable executes, the _type's value is nil.

STEP 2. Pass in and Accept the Child Type

Have your form view send the child_type along with the typical form data, and your controller must permit it in its strong parameters check.

# app/views/polymorph/_form.html.erb
<%= form_for(@polymorph) do |form| %>
  # Pass in the child_type - This one has been turned into a chicken!
  <%= form.hidden_field(:polymorph_type, value: 'Chicken' %>
  ...
  # Form values for Chicken
  <%= form.fields_for(:chicken) do |chicken_form| %>
    <%= chicken_form.text_field(:hunger_level) %>
    <%= chicken_form.text_field(:poop_level) %>
    ...etc...
  <% end %>
<% end %>

# app/controllers/polymorph_controllers.erb
...
private
  def polymorph_params
    params.require(:polymorph).permit(:id, :polymorph_id, :polymorph_type)
  end

Of course, your view(s) will need to handle the different types of models that are 'associable', but this demonstrates one.

Hope this helps someone out there. (Why do you need polymorphic chickens anyway?)

accepts_nested_attributes_for with belongs_to polymorphic

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