Derive update fields from the ActiveAdmin form block

[lloydwatkin]

Summary

The MCP update tool could not update any record on resources that declare their writable fields through a form do … end block instead of permit_params. RecordUpdater#permitted_attributes resolved fields solely from the controller's permitted_params; for a form-only resource ActiveAdmin's default permitted_params returns nil, so the tool crashed with:

Could not determine permitted attributes: undefined method '[]' for nil

This blocks a very common ActiveAdmin style — e.g. the consumer that surfaced this has 141 registered resources, only 17 of which declare permit_params while ~92 declare a form block.

Change

• Prefer the resource's permit_params when it's declared (unchanged behaviour).
• Otherwise derive the permitted fields from the resource's form block. A new FormFieldCollector runs the form block against a recording stand-in form builder: every input :field records :field; inputs/actions/semantic_errors/helper calls and any other message are swallowed (returning self) so arbitrary Formtastic DSL — as: :hidden, collection: Country.pluck(:name), conditionals, select2 — executes without a real view context. has_many associations are intentionally not descended into (the updater only writes flat attributes).
• Fail closed: when neither permit_params nor a form block can be resolved, raise rather than silently permitting all columns.

Verification

• bundle exec rspec — 63 examples, 0 failures (7 new specs covering the form-derivation path, arbitrary DSL tolerance, and both fail-closed cases).
• Prototyped the collector against a real ActiveAdmin Users::Address resource (whose form calls Country.order(:name).pluck(:name), as: :hidden, select2): it extracts exactly the 12 form fields with no errors.

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