nested attributes - Retrieving form params in Rails 4 controller using fields_for and accepts_nested_attributes_for -

i positive dumbest question cannot wrap head around it.

i have 2 models in simple has_one/belongs_to relationship

registration_code.rb

class registrationcode < activerecord::base   has_one :billing_transaction   accepts_nested_attributes_for :billing_transaction 

billing_transaction.rb

class billingtransaction < activerecord::base   belongs_to :registration_code 

in form collecting information both models using fields_for.

_form.html.erb (truncated example)

<%= form_for @registration_code, :html => {:class => "form", role: "form"} |f| %>    <%= f.label :registration_code, "registration code", :class => "control-label" %>   <%= f.text_field :registration_code %>    <%= f.fields_for @billing_transaction |bt|  %>     <%= bt.label :transaction_amount, "transaction amount", :class => "control-label" %>     <%= bt.number_field :transaction_amount %>   <% end %>  <% end %> 

in controller, have following.

registration_code_controller.rb

def new   @registration_code = registrationcode.new   @billing_transaction = billingtransaction.new   @billing_transaction.registration_code = @registration_code end  def create     @registration_code = registrationcode.new(registration_code_params)     @billing_transaction = billingtransaction.new # have take params?     @billing_transaction.registration_code = @registration_code # need line?      ##### trouble on next line #####     @billing_transaction.transaction_amount = params[:billing_transaction_attributes][:transaction_amount] # never gets set!  not sure how access params      respond_to |format|       if @registration_code.save && @billing_transaction.save         format.html { redirect_to registration_codes_path, notice: 'registration code created.' }       else         format.html { render :new }         format.json { render json: @customer.errors, status: :unprocessable_entity }       end     end end  private  # never trust parameters scary internet, allow white list through. def registration_code_params     params.require(:registration_code).permit(:registration_code, :expires_at, billing_transaction_attributes: [:transaction_amount]) end 

the params submitted , can access params primary model (registration_code) fine. cannot life of me figure out how parameters of "sub" model (billing_transaction) , use them in controller.

{"utf8"=>"✓", "authenticity_token"=>"eppqwjketagmzb5wrwei6aycx4xpqvq4rl2m405ibwldbp9xe0rypgtz6nmx8svcfu94gkrfmfv9prokka1blg==",  "registration_code"=>{"registration_code"=>"hfmkbqen", "expires_at"=>"2015-07-16",  "billing_transaction"=>{"transaction_amount"=>"958.40" }},  "commit"=>"create registration code"} 

to access billing_transaction.transaction_amount, example, have tried many variations:

• params[:billing_transaction_attributes][:transaction_amount]
• params[:billing_transaction][:transaction_amount]
• params[@billing_transaction][:transaction_amount]
• params[:registration_code][:billing_transaction][:transaction_amount]

no matter enter cannot seem access nested array of parameters.

help. feeling super dumb right now. thanks.

a few important changes new , create methods below solve problem.

def new   @registration_code = registrationcode.new   @billing_transaction = @registration_code.build_billing_transaction end  def create   @registration_code = registrationcode.new(registration_code_params)    respond_to |format|     if @registration_code.save       format.html { redirect_to registration_codes_path, notice: 'registration code created.' }     else       format.html { render :new }       format.json { render json: @customer.errors, status: :unprocessable_entity }     end   end end 

this how nested_attributes gets saved in db.