How does Rails group arrays of hashes during form submission?

Question

According to the Rails Guides and this Railscasts episode, when there's a one-to-many association between two objects (e.g. `Project` and `Task`), we can submit multiple instances of `Task` together with the `Project` during form submission similar to this: ``` <% form_for :project, :url => projects_path do |f| %>

Name: <%= f.text_field :name %>

<% for task in @project.tasks %> <% fields_for "project[task_attributes][]", task do |task_form| %>

Task Name: <%= task_form.text_field :name %> Task Duration: <%= task_form.text_field :duration %>

<% end %> <% end %>

<%= submit_tag "Create Project" %>

<% end %> ``` This will result multiple copies of an HTML block like this in the form, one for each task: ```

Task Name: Task Duration:

``` My question is, how does Rails understand which ``` (project[task_attributes][name], project[task_attributes][duration]) ``` bel…

Accepted Answer

Yes, ordering is retained as-is, as @k-everest self-answered as a comment to original question.

Those asking for HTML, see the guide on how the attribute's `name` is parsed.

Example of typically bad ordering:

```
cart[items][][id]=5
cart[items][][id]=6
cart[items][][name]=i1
cart[items][][name]=i2
```

And that gets parsed by Rails into this:

```
{ "cart"=> {"items"=> [
                        {"id"=>"5"},
                        {"id"=>"6", "name"=>"i1"},
                        {"name"=>"i2"}
                       ]}}
```

Example source: https://spin.atomicobject.com/2012/07/11/get-and-post-parameter-parsing-in-rails-2/

The feature was added in Rails' initial commit, with method name `build_deep_hash`. For more history, skip the flaming/semantics war and go for the last post from the end here: https://www.ruby-forum.com/topic/215584

How does Rails group arrays of hashes during form submission?

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