Partial date-property TTL updates reset an omitted offset to zero #2170

Description

Reconfigure.ObjectTTL.delete_by_date_property() changes an omitted ttl_offset to 0. A call that only supplies filter_expired_objects consequently includes an explicit defaultTtl: 0 in the generated TTL update block. Other TTL update helpers leave an omitted offset as None.

Reproduction

This inspects the TTL update block locally; it does not test the complete collection update request or server behavior:

from weaviate.classes.config import Reconfigure

update = Reconfigure.ObjectTTL.delete_by_date_property(filter_expired_objects=True)
print(update.defaultTtl)
print(update.merge_with_existing({}))
# Actual: 0 and {'enabled': True, 'filterExpiredObjects': True, 'defaultTtl': 0}
# Expected: None and {'enabled': True, 'filterExpiredObjects': True}

existing = {
    'enabled': True,
    'filterExpiredObjects': False,
    'deleteOn': 'expiresAt',
    'defaultTtl': 3600,
}
print(update.merge_with_existing(existing))

The direct TTL-block merge overwrites defaultTtl with zero even though no offset was supplied. An explicit ttl_offset=0 should still reset it. Negative existing offsets have the same issue.

Scope clarification from review

Review on #2171 identified a separate casing mismatch: the collection update path uses objectTTLConfig, while creation and reading use objectTtlConfig. A local collection merge with the latter spelling produces two blocks. The original reproduction above operates on an individual TTL block and cannot establish server-side preservation. The delete_by_* factories also set enabled=True, even for a filter-only call. Those existing behaviors need a maintainer decision before the offset fix lands.

Environment and validation

Originally reproduced on main commit 142d798, Python 3.13.13, macOS arm64. The offset coercion and related collection-key/activation paths remain present in upstream eb5546a8 inspected on 2026-09-29.

PR #2171 leaves None unchanged in the update factory; creation still defaults to zero. Revised regression tests verify offset omission directly, positive/negative TTL-block merges, explicit integer/timedelta offsets including zero, and the creation default. Restoring the coercion gives two failures; with the fix both configuration test modules pass (233 tests). No live-server integration test was run.

AI assistance was used to investigate, implement, and test this patch and review follow-up.