Теория: Nested Forms

form_for @user do |f|
  # вложенная форма
  = f.simple_fields_for :posts do |posts_form|
    = posts_form.input :title
  end
end

form_for @user do |f|
  # вложенная форма
  = f.simple_fields_for :posts do |posts_form|
  = render 'posts_field', posts_form: posts_form
  end
end

class Project < ActiveRecord::Base
  has_many :tasks, inverse_of: :project, dependent: :destroy
  accepts_nested_attributes_for :tasks, reject_if: :all_blank, allow_destroy: true
end

class Task < ActiveRecord::Base
  belongs_to :project
end

= simple_form_for @project do |f|
  = f.input :name
  = f.input :description
  %h3 Tasks
  #tasks
    = f.simple_fields_for :tasks do |task|
      = render 'task_fields', f: task
    .links
    # Нажатие по ссылке будет подгружать на страницу новые поля
      = link_to_add_association t('.add_task'), f, :tasks
  = f.submit

.nested-fields
  = f.input :description
  = f.input :done, as: :boolean
  # Нажатие по ссылке будет удалять поля
  = link_to_remove_association t('.remove_task'), f