Inertia forceFormData sends indexed keys ([0], [1]) instead of [], causing issues with Rails strong params

[florianfelsing]

Problem

I'm encountering an issue with Inertia's forceFormData behavior when submitting file arrays in a Rails app:

• @inertiajs/react: 2.0.17
• inertia-rails: 3.10

When submitting a payload like:

{
  product: {
    name: "Test",
    images: [File1, File2, File3]
  }
}

with:

router.post('/products', data, { forceFormData: true })

Inertia serializes it into indexed keys:

product[images][0]: <File1>
product[images][1]: <File2>
product[images][2]: <File3>

Rails parses this into:

params[:product][:images] # => { "0" => ..., "1" => ..., ... }

…which fails the strong parameters declaration:

params.require(:product).permit(:name, images: [])

The images field gets filtered out and validation errors follow.

Expected Behavior

FormData keys should be serialized as:

product[images][]: <File1>
product[images][]: <File2>

This results in a valid array in Rails:

params[:product][:images] # => [<UploadedFile>, <UploadedFile>, ...]

Temporary Workaround

We’re currently patching the input server-side:

def normalize_images_param
    images = params.dig(:product, :images)
    return unless images.is_a?(ActionController::Parameters)

    params[:product][:images] = images.values
  end

This converts the hash back into an array before permitting the field.

[skryukov]

Yup, that's a known issue: inertiajs/inertia#1901. Probably I can simply reuse the same option Inertia uses atm and call it a bugfix instead of introducing a new one. I will do that in the next couple of days and see if that helps the PR get merged 😅