Q:

pass array hidden field in nested model rails

I have following code in my view:
    <% @m1.map(&:id).each do |id|%>

    <%= b.fields_for :modul1hours do |f| %>

    <%= f.hidden_field :modul1_id, id %>
    <%= f.text_field :module_est_hours, :size => 30 %>
    </tr>
  <% end %>

<%end%>

params passing in console
Parameters: {"authenticity_token"=>"LJ/ZME2lHZ7VwCDgPKX6OFe326fXSXo5UB4M0cPwbCE=", "esthour"=>{"rfp_id"=>"6", "ecommerce_est_hours"=>"", "modul1hours"=>{"module_est_hours"=>"3"}, "designpages_est_hours"=>"", "cms_est_hours"=>""}, "modul1_ids"=>["12", "13", "14"], "utf8"=>"✓", "project_id"=>"second", "commit"=>"Add Todo"}

Current user: admin (id=1)
modul1_ids is the hidden array based on that three text box is created but when i submit the page gives me:

ActionView::Template::Error (undefined method `merge' for 12:Fixnum):

in first textbox i passed 1
    second                2
    and in third          3
last value(3) isthe s passing that one can see in the console params module_est_hours"=>"3, but what about rest two fields y not passing and whats the solution for an error. Please help me.
Edit 1
    <% @m1.map(&:id).each do |id|%>

    <%= b.fields_for :modul1hours do |f| %>

    <%= hidden_field_tag "modul1_ids[]", id %>
    <%= f.text_field :module_est_hours, :size => 30 %>
    </tr>
  <% end %>

<%end%>

this code does not give the error, but also value is not stored in modul1hours table
The field of the modul1hours table are:
integer :modul1_id
decimal :module_est_hours
decimal :module_act_hours
integer :esthours_id

]
.rb
belongs_to :esthour
attr_accessible :module_est_hours,:module_act_hours
and controller
Update
    def new
@esthour = Esthour.new
 @gg =  @esthour.modul1hours.build
@project = params[:project_id]
@rfp = params[:rfp_id]

@m1 = Modul1.where(:rfp_id => @rfp.id)

respond_to do |format|
  format.html # new.html.erb
  format.json { render :json => @esthour }
end

end
over Update
  # GET /project_todos/1/edit
      def edit
      @esthour = Esthour.find(params[:id])
      end

           def create

            @project = params[:project_id]

            @esthour = Esthour.new(params[:esthour])

            user_params = params.select{|k,v| k.include?('esthour')}

            respond_to do |format|

           if @esthour.save

       get_issue_attribute_param1(user_params)

             format.html { redirect_to project_rfp_url(@project,@esthour.rfp_id), :notice => 'hours was successfully created.' }
            format.json { render :json => @esthour, :status => :created, :location => @esthour }
  else
            format.html { render :action => "new" }
            format.json { render :json => @esthour.errors, :status => :unprocessable_entity }
  end
end
 end

is there any build needed?eg Esthour.modul1hour.build in new def of controller coz record not saved in table? 
view
    <%= form_for @esthour,:rfp_id => @rfp.id,:project_id => @project do |b| %>

    <%= b.hidden_field :rfp_id, :value => @rfp.id %>

    <%= hidden_field_tag :project_id, @project %>
   <table>
      <tr>    <td><b>Menutype </b></td>
    <% if  @rfp.menutype.present? %>
        <td><%= @rfp.menutype %></td> 
        <td><%= b.number_field :menutype_est_hours %></td>
    <% end %>
</tr>

      <tr>           <td> <b>Number of menu</b> </td>
    <% if  @rfp.numberofmenu.present? %>
        <td><%= @rfp.numberofmenu %></td> 
        <td><%= b.number_field :numberofmenu_est_hours %></td>
    <% end %>
   </tr>

    <tr>

        <% @m1.map(&:id).each do |id|%>

         <%= b.fields_for :modul1hours do |f| %>

          <%= f.hidden_field :modul1_id, value => id  %>
          <%= f.text_field :module_est_hours, :size => 30 %>
         </tr>
      <% end %>

     <% end %>

      </table>
     <%= b.submit 'Add Todo' %>
     <% end %>

     @esthour = Esthour.new

         @gg =  @esthour.modul1hours.build
       @project = params[:project_id]

A:

In this line:
<%= f.hidden_field :modul1_id, id %>

You are saying that you want the hidden field binded with modul1hour modul1_id method and options being id. Second parameter for FormBuilder hidden_field is expected to be a hash (which is then merged against default options). To do what you want do:
<%= f.hidden_field :modul1_id, value: id %>

A:

Hidden fields aren't really the issue here
Apart from @BroiStatse's answer, I can see the issue as how you handle the params on your controller

Nested Models
Sending data to a controller sends that data to the relevant models. This is normally handled with accepts_nested_attributes_for, but can be handled manually too
From your controller code, I can't see how you're dealing with your extra data, but your error is caused by the incorrect merge of the params
Instead of saving the data manually, I would use the accepts_nested_attributes_for to save the data, like this:
#app/models/project.rb
Class Project < ActiveRecord::Base
    accepts_nested_attributes_for :modul1hours
end

This will pass the params to your modul1hours model, where you'll then have to capture them with the relevant attr_accessible actions

f.fields_for
In order to get accepts_nested_attributes_for working properly, you have to ensure you use the f.fields_for function correctly.
You have to first build the ActiveRecord objects in your new controller action, like this:
def new
    @project = Project.new
    @project.modul1hours.build
end

Your problem is that you're then cycling through the ID's of your modul1hours model, yielding the f.fields_for artificially. Rails will only output an f.fields_for if the ActiveRecord object has been built in the controller:

  
    
     "30" %>
  

This RailsCast gives you a better idea about this
What I would do is this:
#app/controllers/projects_controller.rb
def new
    @project = Project.new
    @m1.map(&:id).each do |i|
        @project.modul1hours.build
    end
end

#app/views/projects/new.html.erb
<%= b.fields_for :modul1hours do |f| %>
    <%= hidden_field_tag :id, value :id %>
    <%= f.text_field :module_est_hours, :size => "30" %>
<% end %>

I'm still thinking about how I would assign the ID's to the hidden field

Update
Try this:
#app/controllers/projects_controller.rb
def new
    @project = Project.new
    @project.modul1hours.build
end

Replace modul1hours with whatever your projects has_many of 

