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    <p>I've been beating my head against this one on and off for years, and I just today came up with something I'm proud of - elegant, unobtrusive, and you can do it only two or three (really long) lines of code.</p> <p>ParentFoo has_many NestedBars, with appropriate accepts_nested_parameters and all those goodies. You render a set of fields for a nested_bar into a data-add-nested attribute on the link, using the :child_index to set a string that you can replace later. You pass that string in a second parameter, data-add-nested-replace</p> <p>parent_foos/_form.html.haml</p> <pre><code>- semantic_form_for @parent_foo do |f| -# render existing records - f.semantic_fields_for :nested_bars do |i| = render 'nested_bar_fields', :f =&gt; i # magic! - reuse_fields = f.semantic_fields_for :nested_bars, @parent_foo.nested_bars.build, :child_index =&gt; 'new_nested_bar_fields' do |n| render('nested_bar_fields', :f =&gt; n) end = link_to 'Add nested bar', '#', 'data-add-nested' =&gt; reuse_fields, 'data-add-nested-replace' =&gt; 'new_nested_bar_fields' </code></pre> <p>the nested fields partial is nothing fancy</p> <p>parent_foos/_nested_bar_fields.html.haml</p> <pre><code>- f.inputs do = f.input :name = f.input :_delete, :as =&gt; :boolean </code></pre> <p>In your jQuery you bind the click of elements with a data-add-nested field (I've used live() here so ajax-loaded forms will work) to insert the fields into the DOM, replacing the string with a new ID. Here I'm doing the simple thing of inserting the new fields before() the link, but you could also provide an add-nested-replace-target attribute on the link saying where in the DOM you want the new fields to wind up.</p> <p>application.js</p> <pre><code>$('a[data-add-nested]').live('click', function(){ var regexp = new RegExp($(this).data('add-nested-replace'), "g") $($(this).data('add-nested').replace(regexp, (new Date).getTime())).insertBefore($(this)) }) </code></pre> <p>You could put this in a helper, of course; I'm presenting it here directly in the view so the principle is clear without mucking about in metaprogramming. If you're worried about names colliding, you could generate a unique ID in that helper and pass it in nested-replace.</p> <p>getting fancy with the delete behaviour is left as an exercise for the reader.</p> <p><em>(shown for Rails 2 because that's the site I'm working on today - almost the same in Rails 3)</em></p>
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    1. POHow to add and remove nested model fields dynamically using Haml and Formtastic
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    1. COnote: it may not be that easy in Rails 3, thanks to html escaping :(
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    2. COusing escape_once seems to work, as in this semi-related example with inserting a single field: - new_field = escape_once f.input(:related_thing) = link_to 'Add another', '#', 'data-insert' => new_field
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    3. COI've developed this technique a little further for use without a model - I had a case where a client wanted to specify sets of fields to show in a report. This version uses a block instead of a separate template for the fields, and uses a 'generic model' to generate the replaceable parameter. Here's the gist: https://gist.github.com/1554795
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