Using observe_field on an field inside a fields_for

rubyonrails-talk
1

I'm trying to observe a field that get generated inside a fields_for loop (I'm trying to create the form dynamically depending on a selection value). Is there a way to access the index in the field_for loop?

Thanks, Jay

2

Hi Jay,

3

OK, here's an example of what I'm trying to do.

_form.html.erb

<%= form.error_messages %> <p>   <%= form.label :title %><br />   <%= form.text_field :title %> </p> <p>   <%= form.label :due_date %><br />   <%= form.datetime_select :due_date %> </p> <div id="tasks">   <h3>Tasks</h3>   <% form.fields_for :tasks do |task_form| -%>     <%= render :partial => 'task', :locals => { :form => task_form } %>   <% end -%> </div> <%= add_task_link(form) %> <p>   <%= form.submit 'Save' %> </p>

And here's the partial that gets called above (_task.html.erb)

<div class="task">   <p>     <%= form.label :name %>     <%= form.text_field :name, :size => 15 %>     <%= remove_task_link( form ) %>   </p>   <%= observe_field :project_tasks_attributes_0_name, :function => 'alert("field changed")' %> </div>

This doesn't work because I need to change the observe_field name (:project_tasks_attributes_0_name) for each task. If I had 3 tasks, the then the observe_field names would be: :project_tasks_attributes_0_name :project_tasks_attributes_1_name :project_tasks_attributes_2_name

I was wondering if there is a way to access the index of the task form loop to generate field names like the above.

Thanks, Jay

4

Hi Jay,

OK, here's an example of what I'm trying to do.

_form.html.erb

<%= form.error_messages %> <p>   <%= form.label :title %><br />   <%= form.text_field :title %> </p> <p>   <%= form.label :due_date %><br />   <%= form.datetime_select :due_date %> </p> <div id="tasks">   <h3>Tasks</h3>   <% form.fields_for :tasks do |task_form| -%>     <%= render :partial => 'task', :locals => { :form => task_form } %>   <% end -%> </div> <%= add_task_link(form) %> <p>   <%= form.submit 'Save' %> </p>

And here's the partial that gets called above (_task.html.erb)

<div class="task">   <p>     <%= form.label :name %>     <%= form.text_field :name, :size => 15 %>     <%= remove_task_link( form ) %>   </p>   <%= observe_field :project_tasks_attributes_0_name, :function => 'alert("field changed")' %> </div>

This doesn't work because I need to change the observe_field name (:project_tasks_attributes_0_name) for each task. If I had 3 tasks, the then the observe_field names would be: :project_tasks_attributes_0_name :project_tasks_attributes_1_name :project_tasks_attributes_2_name

I was wondering if there is a way to access the index of the task form loop to generate field names like the above.

Good explanation. Thanks. A couple of suggestions re: approach. The first probably won't work but it would be interesting to know. I wonder if

do |task_form|

could be replaced with

each_with_index |task_form, i|

both do and each take blocks. Probably won't work, but...

If you're sure of the naming / order, then the easiest way would be something like (not tested)...

<% index = 0 %> <% form.fields_for :tasks do |task_form| -%> <%= render :partial => 'task', :locals => { :form => task_form, :index => index } %>   <% index += 1 %> <% end -%>

And then in the partial, using a string instead of a symbol...

<%= observe_field "project_tasks_attributes_#{index}_name" ...

Not terribly rubyish, I know, but something on that order should be workable :wink:

HTH, Bill

5

Hi Jay,

I've just run into exactly the same problem, as I'm using jQuery to build autocompleters and need the generated field id for my observe_field call. I've looked all over for a way to access the index, but couldn't find it anywhere. I decided to delve into the Rails code and found a couple of helper methods in the InstanceTagMethods moduke (http://github.com/rails/rails/blob/master/ actionpack/lib/action_view/helpers/form_helper.rb).

The methods of interest are santized_object_name and sanitized_method_name. They take the FormHelper's object_name (which is publically accessible) and return the generated form element id (e.g. person[attributes][0][name] becomes person_attributes_0_name). However, these methods are private to InstanceTagMethods, so you can't get at them from within your fields_for block.

The solution I've used is to duplicate these methods in my application_helper.rb, and call them from within my fields_for block:

# app/helpers/application_helper.rb def sanitized_object_name(object_name)   object_name.gsub(/\]\[|[^-a-zA-Z0-9:.]/,"_").sub(/_$/,"") end

def sanitized_method_nam(method_name)   method_name.sub(/\?$/, "") end

def form_tag_id(object_name, method_name)   "#{sanitized_object_name(object_name.to_s)}_#{sanitized_method_name (method_name.to_s)}" end

Then in my fields_for partial:

# app/views/people/_person_attributes.rb # wrapped in person_form,fields_for :attributes do |attributes_form|   <%= f.text_field :name %>   <%= observe_field form_tag_id(f.object_name, :name), ... %>

This code works for index partials (i.e. those with absolute indexes) and those generated with timestamp indexes (I'm using a customised version of Ryan Bates' latest nested form code for Rails 2.3 - http://github.com/ryanb/complex-form-examples). I'll post this code on my site aswell.

Hope this helps! If anyone knows a better way to access the form builder's index, please follow up this post!

Regards, Chris