ENG-3603: Fix custom field updates on privacy declarations

[jpople]

Ticket ENG-3603

Description Of Changes

Fixes a bug where updating custom fields on privacy declarations would fail with either a "duplicate key value violates unique constraint" or "null value in column id violates not-null constraint" error.

Root cause: buildCustomFieldsPayload in useSystemDataUseCrud.ts was incorrectly using the custom field definition ID as the custom field record ID in the upsert payload. Since definition IDs are shared across all declarations, the second declaration's custom fields would collide with the first's on the primary key. Additionally, the declaration matching after save only checked data_use, not name, which could target the wrong declaration when multiple share the same data use.

Companion PR: Requires ethyca/fidesplus PR (linked in ENG-3603) for the backend fix that strips id from the bulk update dict to prevent SET id=NULL when the frontend omits the field.

Code Changes

• Removed id from the upsert payload in buildCustomFieldsPayload — the backend finds existing records by (resource_id, custom_field_definition_id) and auto-generates UUIDs for new ones
• Fixed declaration matching in patchDataUses to use both data_use and name, consistent with the backend's privacy_declaration_logical_id
• Normalized null/empty name comparison in declarationAlreadyExists to match backend behavior (null and "" treated as equivalent)

Steps to Confirm

1. Create a system with two privacy declarations that have different data uses
2. Add custom field definitions for the "privacy declaration" resource type
3. Set custom field values on the first declaration — should save without error
4. Set custom field values on the second declaration — should save without error (previously failed with duplicate key violation)
5. Edit a custom field value on one declaration — should only change that declaration, not the other

Pre-Merge Checklist

• Issue requirements met
• All CI pipelines succeeded
• CHANGELOG.md updated
  • Add a db-migration label
  • Add a high-risk label
  • Updates unreleased work already in Changelog
• UX feedback:
  • No UX review needed
• Followup issues:
  • No followup issues
• Database migrations:
  • No migrations
• Documentation:
  • No documentation updates required

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2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
fides-plus-nightly	Ignored		Apr 24, 2026 3:11am
fides-privacy-center	Ignored		Apr 24, 2026 3:11am

[github-actions]

Title	Lines	Statements	Branches	Functions
admin-ui		6.32% (2799/44262)	5.56% (1402/25174)	4.42% (579/13087)
fides-js		79.39% (2011/2533)	65.99% (1240/1879)	73.09% (345/472)
privacy-center		85.97% (331/385)	81.36% (179/220)	78.87% (56/71)

[kruulik]

Looks good to me!