fix: revalidate nested fields after parent object updates

- Fixes the stale nested field meta described in #2113.
- When `setFieldValue` updates a parent object field like `a`, mounted descendant fields like `a.b` are now revalidated on change as well. This keeps nested field-level errors in sync after programmatic parent object updates.
- Added a regression test covering the reported case where updating `a` from `{ b: 0 }` to `{ b: 1 }` should clear the existing validation error on `a.b`.

Fixes #2113

Author: marc-niclas

.changeset/stale-nested-field-meta.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/form-core': patch
+---
+
+Fix stale nested field errors when setting a parent object field by revalidating mounted descendant fields after the parent value changes.

packages/form-core/src/FormApi.ts

@@ -2284,6 +2284,17 @@ export class FormApi<
     const dontUpdateMeta = opts?.dontUpdateMeta ?? false
     const dontRunListeners = opts?.dontRunListeners ?? false
     const dontValidate = opts?.dontValidate ?? false
+    const fieldString = field.toString()
+    const descendantFields = Object.keys(this.fieldInfo).filter((fieldKey) => {
+      if (fieldKey === fieldString) {
+        return false
+      }
+
+      return (
+        fieldKey.startsWith(`${fieldString}.`) ||
+        fieldKey.startsWith(`${fieldString}[`)
+      )
+    }) as DeepKeys<TFormData>[]
 
     batch(() => {
       if (!dontUpdateMeta) {
@@ -2313,6 +2324,10 @@ export class FormApi<
 
     if (!dontValidate) {
       this.validateField(field, 'change')
+
+      descendantFields.forEach((descendantField) => {
+        this.getFieldInfo(descendantField).instance?.validate('change')
+      })
     }
   }
 

packages/form-core/tests/FormApi.spec.ts

@@ -225,6 +225,40 @@ describe('form api', () => {
     expect(form.getFieldValue('name')).toEqual('other')
   })
 
+  it('should clear nested field errors when setting a parent object field', () => {
+    const form = new FormApi({
+      defaultValues: {
+        a: {
+          b: 0,
+        },
+      },
+    })
+
+    const childField = new FieldApi({
+      form,
+      name: 'a.b',
+      validators: {
+        onChange: ({ value }) =>
+          value > 0 ? undefined : 'Must be greater than 0',
+      },
+    })
+
+    form.mount()
+    childField.mount()
+
+    childField.setValue(0)
+
+    expect(childField.state.meta.errors).toEqual(['Must be greater than 0'])
+
+    form.setFieldValue('a', {
+      b: 1,
+    })
+
+    expect(form.getFieldValue('a.b')).toBe(1)
+    expect(childField.state.meta.errors).toEqual([])
+    expect(childField.state.meta.isValid).toBe(true)
+  })
+
   it("should be dirty after a field's value has been set", () => {
     const form = new FormApi({
       defaultValues: {