Support methods around field policies

[sevenseacat]

Is your feature request related to a problem? Please describe.

I think we discussed this a while ago but I couldn't find a record of an issue I'd created to track it. So here it is!

It would be awesome if there was some support functionality around field policies, to make them really usable within an application.

The scenario I was thinking of was -

eg. I have a resource like Album and a user is allowed to edit the album, but not the cover_image_url field.

So ideally I could -

• show the form to edit the resource, but wrap individual field rendering in a can? check to only show the field if the user is allowed to edit it

• have some kind of helper either before/after form submission, that strips fields that the user can't edit if they happen to be present? At the moment, field policies don't seem to do anything here, I did some quick testing with a field policy setup like this -

  field_policies do
    field_policy :* do
      authorize_if always()
    end

    field_policy :cover_image_url do
      authorize_unless action_type(:update)
    end
  end

And my user was still able to submit a form to update the album, including updating the cover_image_url field.

Alternatively, returning a forbidden error in this scenario would be okay I think.

[zachdaniel]

At the moment, field policies are only for reading. We will have to start with making it even possible to use them for creates/updates. Technically that would be a breaking change if we make policies apply to more action types, so we'd need to make something like write_policy to do that.

After that, we could add some helpers around things like can_write_field or something along those lines.

[sevenseacat]

Okay maybe a discussion for a lot later then :)

[zachdaniel]

I think it would be relatively straightforward to add this functionality (not necessarily the helpers, but supporting field policies on writes), and doing it before 3.0 would be pretty reasonable. It would essentially just be policies that apply only if the field is being changed.

[zachdaniel]

Actually, write_policy makes more sense than overloading the field_policy DSL. So before/after 3.0 won't make much difference.

[jimsynz]

I'm not sure that write_policy makes much sense since policies (not field policies) are read/write agnostic and take the applicability from filters such as action_type. What if we made field policies support action_type and action and just defaulted them to action_type(:read)?

[zachdaniel]

What would the defaulting look like?

field_policy :field, condition do
  ...
end

There wouldn't be a way that we can let them override I think. The other thing I'm thinking about is that the logic around write policies (like the helpers) will be significantly different, i.e "can_edit_field". We'll have to spec it out though. I'll keep this for a "later" thing and we can revisit after 3.0.