In-memory database: read a field as it was at an earlier time

Context

Welcome to the platform team. The in-memory database in database/ handles records, scans and values that expire (levels 1 to 3), and its tests pass. Billing needs to ask what a field held at an earlier moment. The method is stubbed in database/database.py, and the rules for level 4 are in README.md.

Build level 4 in 40 minutes. The checks are graded one by one, so each part you get working counts even if the rest is not done.

MDB-4: Point-in-time reads (level 4)

Acceptance criteria

• get_when(timestamp, key, field, at_timestamp) returns the value left by every call made at or before at_timestamp, including calls made at exactly at_timestamp.
• A field that did not exist yet, had been deleted, or had expired by at_timestamp reads as None.
• Expiry is checked at at_timestamp: a value set with a TTL is seen before it expired, even if get_when is called much later.
• A compare call that returned False changes nothing, so get_when does not see it.
• Levels 1 to 3 keep working.

Notes

• Today the database only keeps the current value of each field. You will need to keep more.
• Run pytest tests/visible. tests/visible/test_level4.py fails until level 4 works.
• Practice task modelled on candidate reports of Meta's four-level in-memory database online assessment. Original StackYard code, not the actual question; not affiliated with Meta.