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    <p>As others have mentioned, the <code>[]</code> should contain a key for new records because otherwise it is mixing a hash with an array type. You can set this with the <code>child_index</code> option on fields_for.</p> <pre><code>f.fields_for :items, Item.new, child_index: "NEW_ITEM" # ... </code></pre> <p>I usually do this using the <code>object_id</code> instead to ensure it is unique in case there are multiple new items.</p> <pre><code>item = Item.new f.fields_for :items, item, child_index: item.object_id # ... </code></pre> <p>Here's an abstract helper method that does this. This assumes there is a partial with the name of <code>item_fields</code> which it will render.</p> <pre><code>def link_to_add_fields(name, f, association) new_object = f.object.send(association).klass.new id = new_object.object_id fields = f.fields_for(association, new_object, child_index: id) do |builder| render(association.to_s.singularize + "_fields", f: builder) end link_to(name, '#', class: "add_fields", data: {id: id, fields: fields.gsub("\n", "")}) end </code></pre> <p>You can use it like this. The arguments are: the name of the link, the parent's form builder, and the name of the association on the parent model.</p> <pre><code>&lt;%= link_to_add_fields "Add Item", f, :items %&gt; </code></pre> <p>And here is some CoffeeScript to listen to the click event of that link, insert the fields, and update the object id with the current time to give it a unique key.</p> <pre><code>jQuery -&gt; $('form').on 'click', '.add_fields', (event) -&gt; time = new Date().getTime() regexp = new RegExp($(this).data('id'), 'g') $(this).before($(this).data('fields').replace(regexp, time)) event.preventDefault() </code></pre> <p>That code is taken from <a href="http://railscasts.com/episodes/196-nested-model-form-revised" rel="noreferrer">this RailsCasts Pro episode</a> which requires a paid subscription. However, there is a full working example freely available <a href="https://github.com/railscasts/196-nested-model-form-revised" rel="noreferrer">on GitHub</a>.</p> <p><strong>Update:</strong> I want to point out that inserting a <code>child_index</code> placeholder is not always necessary. If you do not want to use JavaScript to insert new records dynamically, you can build them up ahead of time:</p> <pre><code>def new @project = Project.new 3.times { @project.items.build } end &lt;%= f.fields_for :items do |builder| %&gt; </code></pre> <p>Rails will automatically insert an index for the new records so it should just work.</p>
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    1. POHow to submit multiple NEW items via Rails 3.2 mass-assignment
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    1. COThis is the answer I've seen a lot, and I'm really bothered by using some pseudo-random number like the object_id as a differentiator. Not to mention: exposing internal object IDs in a client-facing interface? Scary! So many attacks are based on using inadvertantly exposed internals. What really bothers me is that Rails has a perfectly good way to submit arrays of param hases: the '[]'. And if you can coerce it into leaving the '[]' alone, this works beautifully! It just tries really hard not to let you use '[]' in all the field names.
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    2. COThe `object_id` isn't a necessary implementation, you can use any random value there: `id = SecureRandom.hex`. However that does not guarantee it is unique. Perhaps a hash of the object_id would be better?
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    3. COI do a agree that the behavior of inserting the id when passing in `[]` in `fields_for` is odd. AFAIK it has been this way since `fields_for` was first added. You can see that in [this episode](http://railscasts.com/episodes/75-complex-forms-part-3) at about the 3:00.
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