gistfile1.txt

class Task < ActiveRecord::Base
  belongs_to :project
end

project.rb

class Project < ActiveRecord::Base

has_many :tasks
accepts_nested_attributes_for :tasks

end

projects-controller

  # GET /projects/new
  def new
    @project = Project.new
    @project.tasks.build
  end
  
  ...
  
      def project_params
      params.require(:project).permit(:name, tasks_attributes:[:id, :name, :project_id])
    end
  

projects-form.html.erb

<%= form_for(@project) do |f| %>

...

<h4>Tasks<h4>
<%= f.fields_for :tasks do |s| %>
<%= s.text_field :name %>
<% end %>