[imacomber]

Follow-up to #240.

With update_only: true, *_attributes= updates the existing nested model with assign_attributes, which is strict, so an unknown key in the nested payload made the whole cast raise:

Settings.to_type.cast_value(section: { title: "a" }, section_attributes: { title: "b", qq: 1 })
# => ActiveModel::UnknownAttributeError: unknown attribute 'qq' for Section

Now, when the parent is being built by a StoreModel type, the nested model it updates in place collects unknown keys the same way. Assigning to the nested model directly, or from an ActiveRecord parent, is still strict.

🤖 Generated with Claude Code

[DmitryTsepelev]

Thanks for the quick follow-up. The cast case from #240 works now, and CI is green.

I'd like one change in behaviour before merging, explained inline. Right now the same *_attributes= call keeps unknown keys in one situation and raises in another. The other inline comments are small.

Please also add a line for this PR under ## master in CHANGELOG.md. Your author link is already at the bottom of the file from #240.

[DmitryTsepelev]

With update_only, the same assignment now behaves differently depending on whether the nested model already exists.

When there is no supplier yet, *_attributes= goes through send("supplier=", ...), which casts through the type. The type keeps unknown keys. When a supplier exists, this method only keeps them if the parent is being built by a type cast at that moment. In every other case the same call raises:

class Settings
  include StoreModel::Model
  attribute :supplier, Supplier.to_type
  accepts_nested_attributes_for :supplier, update_only: true
end

Settings.new(supplier_attributes: { title: "a", extra: 1 })
# keeps "extra" in supplier.unknown_attributes

settings = Settings.new(supplier: { title: "a" })
settings.supplier_attributes = { extra: 1 }
# => ActiveModel::UnknownAttributeError: unknown attribute 'extra' for Supplier

Master behaves the same way here, so this isn't new. But this PR is the place to make the two paths agree.

I'd make the in-place update always keep unknown keys, the same way the replacement path does. That also means this method no longer needs to read the parent's @collecting_unknown_attributes. That variable belongs to UnknownAttributeAssignment, and NestedAttributes is also mixed into ActiveRecord models, which never set it. So for an ActiveRecord parent the existing-model case is always strict today, and this change would fix that too.

One thing to watch: an ActiveRecord parent can't call a protected method on the nested model, so the nested model will need a public entry point for this. Marking it :nodoc: would keep it out of the docs.

[DmitryTsepelev]

This spec records the split described in the other comment as the intended behaviour. The same assign_attributes(supplier_attributes: { extra: 1 }) succeeds when supplier is nil, and this spec only covers the case where it raises. With the change I suggested, it would become "keeps unknown keys assigned to an existing model", and it would be worth adding an ActiveRecord-parent example next to it.

[DmitryTsepelev]

This method ends up public, because it comes after the class-level private and there's no instance-level private above it. That adds update_nested_model to the public API of every StoreModel and every ActiveRecord model that includes NestedAttributes, and it could clash with a method of the same name there. Could you make it private?

[DmitryTsepelev]

Small naming point. model.collecting_unknown_attributes { ... } reads like a getter, because it has the same name as the instance variable it changes. Something like collect_unknown_attributes { ... } would show it takes a block, and it would match UnknownAttributeAssignment.collect.