Validation error reset my values why - ruby-on-rails-3

Question

I have a modal "A" and it has a line_item model "B" with has_many association. That means B is associated with A. I have validation for B in A's model that
validates_presence_f :B
validates_associated :B
now in my form i have used "fields_for" to save values of B, if i submit a blank form than
validation fails and displays an error message for presence of line item B, but the fields of B's disabled, i have to reshow their fields. Can any one predict why this is happening.
here is my model :
Model A:
class Purchase < ActiveRecord::Base
has_many :purchase_line_items
accepts_nested_attributes_for :purchase_line_items, :reject_if => lambda {|a| a[:account_id].blank? }, :allow_destroy =>true
validates_presence_of :purchase_line_items
validates_associated :purchase_line_items
end
and Model B:
class PurchaseLineItem < ActiveRecord::Base
belongs_to :purchase
end
in my controller:
class PurchasesController < ApplicationController
def new
#purchase = Purchase.new
#purchase.purchase_line_items.build
respond_to do |format|
format.html # new.html.erb
format.xml { render :xml => #purchase }
end
end
end
and in last my views :
<%= form_for #purchase, :html => {:multipart => true} do |f| %>
<%= render 'shared/form_error', :object => #purchase %>
<% #purchase.purchase_line_items.each_with_index do |purchase_line_item, index| %>
<%= render "purchase_line_items", :purchase_line_item => purchase_line_item, :index => index %>
<% end %>
<%= f.submit %>
<% end %>
And in line item partial i have:
<tr id="row<%= index %>" valign="top" >
<%= hidden_field_tag "purchase[purchase_line_items_attributes][#{index}][id]",purchase_line_item.id%>
<td valign="top">
<%= select_tag "purchase[purchase_line_items_attributes][#{index}][account_id]", options_from_collection_for_select(#to_accounts, :id, :name,:selected => purchase_line_item.account_id ), :include_blank => true, :class=>"full" %>
</td>
<td><%= text_field_tag "purchase[purchase_line_items_attributes][#{index}][amount]", purchase_line_item.amount, :class => 'full', :id => 'total', :readonly => 'readonly', :size => 5%></td>
<td><%= link_to image_tag("/images/black_icon/ic_cancel.png"),{:action => :remove_line_item, :index => index}, :remote => true unless index == 0 %></td>
</tr>

Answer 1

If I have correctly understood you, you have problems with nested form.
For what reason do you have readonly property for :amount field?
I do not know, why have you used approach without fields_for, but In this case you can not validate nested fields without it. So your code has to be something like this:
<%= form_for #purchase, :html => {:multipart => true} do |f| %>
<%= render 'shared/form_error', :object => #purchase %>
<%= f.fields_for :purchase_line_items do |pl| %>
<tr id="row<%= pl.object.id %>" valign="top" >
<%= pl.hidden_field :id %>
<td valign="top">
<%= pl.select :account_id, options_from_collection_for_select(#to_accounts, :id, :name, :selected => pl.object.account_id), :include_blank => true, :class=>"full" %>
</td>
<td><%= pl.text_field :amount, :class => 'full', :id => 'total', :readonly => 'readonly', :size => 5 %></td>
<td><%= link_to image_tag("/images/black_icon/ic_cancel.png"), {:action => :remove_line_item, :index => pl.object.id }, :remote => true unless index == 0 %></td>
</tr>
<% end %>
<%= f.submit %>
<% end %>
Add validation to your PurchaseLineItem model to prevent saving record without :account_id.
class PurchaseLineItem < ActiveRecord::Base
belongs_to :purchase
validates_presence_of :account_id
end
Actually if you will validate :account_id you do not have to use :reject_if.
class Purchase < ActiveRecord::Base
has_many :purchase_line_items
accepts_nested_attributes_for :purchase_line_items, :allow_destroy =>true
validates_presence_of :purchase_line_items
validates_associated :purchase_line_items
end
To be more clear, please, edit your question and add code of createaction.