I am using Rails 3.0.9 and Ruby 1.9.2

Models associations are

document.rb
    has_many :sections
    accepts_nested_attributes_for :sections, :allow_destroy => :true,
          :reject_if => proc { |attrs| attrs.all? { |k, v| v.blank? } }section.rb
    belongs_to :document
    has_many :paragraphs, :dependent => :destroy
    has_many :contents :through => :paragraphs
    validates :user_id, :presence =>  { :message => "Must be filled" }

paragraph.rb
    attr_accessible :user_id, :section_id, :content_id
    belongs_to :section
    belongs_to :content
    validates :user_id, :section, :content, :presence =>  { :message =>
                                                  "Must be filled" }

paragraphs table just like a intermediate table for sections and
contents and I want to save records in documents, sections and
paragraphs table using single form submission.

_form.html.erb

    <%= form_for @document, :validate => true do |f| %>
     <%= f.error_messages %>
     <div><%= f.text_field :name %></div>

     <% f.fields_for :sections do |builder| %>
       <div><%= builder.text_field :name %></div>
       <div>
        <%= builder.select :content_ids,
         Content.all.collect {|p| [ p.name, p.id ] },
         {:prompt => "Please Select"}, {:class => "nhs_select",
            :multiple => true}  %>
        </div>
        <% end %>
        <%= f.submit :submit%>
   <% end %>


Example parameters when submiting the form
    {"document"=>{"name"=>"sdf",
"sections_attributes"=>{"0"=>{"name"=>"sdf", "description"=>"sdf",
"_destroy"=>"0", "content_ids" => ["1", "2"]}}, "commit"=>"Create
Document"}

In additionally, I should need to update current_user's id to user_id
column of paragraphs table.

def create
    @document = Document.new
    @document.attributes = params[:document]
    @document.sections.each {|section|
      section.user_id = current_user.id
      section.paragraphs.each {|paragraph| paragraph.user_id =
current_user.id}
    }
    if @document.save!
      # success
    else
      render :action => 'new'
    end
end

Problem 1:

At first, I submit the form, rails render new form without error message
even I have implemented code to display error messages.

then again clicked submit button, action goes to update method. But it
should go to new action.

I inspected in console, the inserted records were rollbacked but still
the ID is retain in object.

Example: <form id="edit_document_6" class="edit_document" method="post"
action="/documents/6" accept-charset="UTF-8">

Problem 2:

Documentation said that the changes are not saved to the database when
assigning the attributes as like user.attributes = {:name => °»Rob°…}.

but my case validation is triggered when assigning the attributes so I
can't assign the value to user_id column in paragraph object before
calling save method

@document.attributes = {"sections_attributes"=>{"0"=>{"name"=>"sdf",
"content_ids" => ["1", "2"]}}
or
section = Section.first
section.attributes = {"content_ids" => ["1", "2"]}

How to assign the value to user_id in paragraph object before calling
save method