Nested form_for in rails

I have Test model and User model. Test model is having many users. Test controller is as below.

class TestController
   def create
      Test.create(testparams)
   end

   private
   def testparams
      params.require(:test).permit(:test_name,user_attributes:[:user_name])
   end
end

In the above code new Test would be created. I want to create new users for a existing test.How to do that??

You should be able to apply the same principles. Below is a basic framework which you will have to alter depending on your requirements.

test model

accepts_nested_attributes_for :users, allow_destroy: true

tests_controller

def edit
  @test = Test.find(params["id"]
  @test.users.build
end

def update
  @test = Test.find(params["id"]
  @test.update(testparams)
end

test view

<%= form_for @test do |f| %>
  <%= f.text_field :test_name %>

  <%= f.fields_for :users do |uf| %>
    <%= uf.text_field :user_name %>
  <% end %>
<% end %>