accept nested attributes for has_many & has_many through in same form_for

class User < ActiveRecord::Base
  has_many :tasks
  has_many :task_items, through: :task
  accepts_nested_attributes_for :task
  accepts_nested_attributes_for :task_item
end

class Task < ActiveRecord::Base
 belongs_to :user
 has_many :task_item
end

class TaskItem < ActiveRecord::Base
  belongs_to :task
end

I am able to get & save form data for Task using form_for in the user form.

<%= fields_for :user, user do |f| -%>
 <%= f.fields_for :task do |builder| %>
  <%=builder.text_field :name%>
<%end%>
<% end %>

I want to accept attributes for Task as well as TaskItem in the User form itself using form_for. Unable to figure out how to do this.

I tried with:

<%= fields_for :user, user do |f| -%>
 <%= f.fields_for :task do |builder| %>
   <%=builder.text_field :name%>
   <%= f.fields_for builder.object.checklist do |builder_1| %>
     <%builder_1.object.each do |bb|%>
      <%= bb.check_box :completed%>
     <%end%>
   <%end%>
 <%end%>

It gives undefined method `check_box' for # I want to be able to create a User,its task & task item records all using one form. Any solutions are welcome.

You have many pluralization errors. Check these changes:

class User < ActiveRecord::Base
  has_many :tasks
  has_many :task_items, through: :tasks     #not task
  accepts_nested_attributes_for :tasks      #not task
  accepts_nested_attributes_for :task_items #not task_item
end

class Task < ActiveRecord::Base
 belongs_to :user
 has_many :task_items #not task_item
end

class TaskItem < ActiveRecord::Base
  belongs_to :task
end

Then, the view:

<%= fields_for :user, user do |f| %>
  <%= f.fields_for :tasks do |builder| %>
    <%= builder.text_field :name %>
    <%= builder.fields_for :task_items do |ti| %>
      <%= ti.check_box :completed %>
    <% end %>
  <% end %>
<% end %>