fix(polymorphic): skip pre-delete FK nullify when datasource cascades

[bexchauveto]

Summary

Fixes #289. The agent's delete_records action pre-emptively UPDATEs polymorphic child rows to NULL both the FK id and type column before issuing the parent delete. For ActiveRecord parents that declare dependent: :destroy, this breaks deletion entirely:

• Rails 5+ belongs_to :foo, polymorphic: true adds a presence validator → the UPDATE fails with Validation failed: <relation> must exist.
• A null: false FK column → the UPDATE violates the NOT NULL constraint.
• Even when the FK is nullable, the manual nullify defeats dependent: :destroy: by the time Rails' destroy callback runs, its WHERE notable_id = ? AND notable_type = ? matches zero rows, silently orphaning the children.

The reporter showed the bug works correctly from the Rails console (Rails handles the cascade), but fails through Forest because of this pre-delete UPDATE.

Fix

Plumb a cascade_on_delete flag through to the polymorphic schemas; when true, the agent skips its manual nullify and lets the datasource handle the cascade itself.

• Toolkit — added cascade_on_delete (default false) to PolymorphicOneToManySchema and PolymorphicOneToOneSchema. The flag rides through the RPC layer for free (**schema reconstruction + .to_json serializer dumps all instance vars).
• AR datasource — sets cascade_on_delete: true when the association declares dependent: :destroy, :destroy_async, or :delete_all.
• Agent route — delete_records skips the polymorphic-cleanup loop body when field_schema.cascade_on_delete is truthy. Guarded with respond_to? for compat with anything the customizer might inject.

Mongoid / Snowflake / RPC keep the existing behavior (cascade_on_delete defaults to false, so the manual cleanup still runs).

Test plan

• forest_admin_datasource_toolkit rspec — 463 examples, 0 failures (added defaults + explicit-flag specs on both schemas)
• forest_admin_datasource_active_record rspec — 147 examples, 0 failures (added: cascade_on_delete=true for Project.has_many :members, as: :memberable, dependent: :destroy; =false for User.has_one :address, as: :addressable)
• forest_admin_agent rspec — 683 examples, 0 failures (new with polymorphic relation that cascades on delete context: foreign collection's update is NOT called, parent delete still is)
• forest_admin_datasource_rpc rspec — 160 examples, 0 failures (sanity: schema round-trips through RPC unchanged)
• End-to-end repro against a Rails 8.1 / PG 15 app: a Customer with has_many :notes, as: :notable, dependent: :destroy and notable_id/notable_type null: false — delete now performs the same SQL as customer.destroy! (children deleted via cascade, then parent), instead of erroring on the nullify.

Note

Skip FK nullification before delete for polymorphic relations when datasource cascades

• Adds a cascade_on_delete flag (default false) to PolymorphicOneToManySchema and PolymorphicOneToOneSchema in the toolkit.
• The ActiveRecord datasource sets this flag based on the association's dependent option (:destroy, :destroy_async, or :delete_all) via a new cascade_on_delete? helper in Collection.
• The delete route in Delete#delete_records now skips the pre-delete FK nullify step for polymorphic relations when cascade_on_delete is true, avoiding redundant or conflicting updates before the DB cascade fires.

^{Macroscope summarized 198e837.}

[qltysh]

6 new issues

Tool	Category	Rule	Count
qlty	Duplication	Found 46 lines of similar code in 2 locations (mass = 241)	4
qlty	Structure	Function with many parameters (count = 6): initialize	2

[matthv]

LGTM

[forest-bot]

🎉 This PR is included in version 1.27.1 🎉

The release is available on GitHub release

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