Analysis: Remove in-memory list cost cache from Gyrinx

in-memory-cache-removal-plan.md

Plan: Remove In-Memory List Cost Cache

Background

The codebase has TWO cost caching mechanisms:

1. Database fields (rating_current, stash_current, dirty) - KEEP - central to facts system
2. In-memory Django cache (caches["core_list_cache"]) - REMOVE - no longer needed

The in-memory cache is updated by signals on every List/ListFighter/Assignment change. These signals are expensive and redundant now that the facts system uses DB fields.

How The Systems Work

Facts System (DB fields):

• facts() returns cached DB values if dirty=False
• facts_from_db(update=True) recalculates and updates DB fields
• facts_with_fallback() tries facts() first, falls back to calculation
• Display methods (cost_display, rating_display) use facts when can_use_facts() is True

In-Memory Cache (to be removed):

• update_cost_cache() writes to caches["core_list_cache"]
• cost_int_cached property reads from cache
• Signals fire on every List/ListFighter/Assignment change
• Falls back to expensive cost_int() recalculation if cache miss

Why Remove It

Once all lists have ListAction tracking and views prefetch latest_actions:

• can_use_facts() always returns True
• Display methods use facts() which reads DB fields
• In-memory cache is never read
• Signal overhead becomes pure waste

Prerequisites Before Signal Removal

1. Fix Views Missing with_related_data() Prefetch

Without latest_actions prefetch, can_use_facts() returns False and code falls back to the in-memory cache.

can_use_facts() implementation:

def can_use_facts(self) -> bool:
    if hasattr(self, "latest_actions"):
        return bool(self.latest_actions)
    return False

High Priority Gaps:

File	Line	Issue
gyrinx/core/views/__init__.py	66-68	Dashboard user lists - missing prefetch
gyrinx/core/views/__init__.py	85-93	Dashboard campaign gangs - missing prefetch
gyrinx/core/views/list.py	171-173	ListsListView - missing prefetch
gyrinx/core/views/campaign.py	473-477	Available lists for campaign
gyrinx/core/views/__init__.py	251-253	User profile public lists

Fix Pattern:

# Before
List.objects.filter(...).select_related("content_house")

# After
List.objects.filter(...).with_related_data(with_latest_action=True)

2. Run Backfill in Production

All lists need at least one ListAction so can_use_facts() returns True.

• Enable FEATURE_LIST_ACTION_CREATE_INITIAL=True (already done)
• Use admin filter "Missing action tracking" to find lists without actions
• Run "Initialize action tracking" admin action on all lists

Phase 1: Fix Prefetch Gaps

Files to modify:

• gyrinx/core/views/__init__.py - Dashboard and profile views
• gyrinx/core/views/list.py - ListsListView
• gyrinx/core/views/campaign.py - Campaign list selection

Phase 2: Remove In-Memory Cache Signals

After backfill is complete, remove these signals from gyrinx/core/models/list.py:

Signal	Line	Trigger
update_list_cost_cache_from_list_change	~1165	List post_save, m2m_changed
update_list_cost_cache	~3350	ListFighter pre_delete, post_save, m2m_changed
update_list_cache_for_assignment	~4244	ListFighterEquipmentAssignment post_delete, post_save
update_list_cache_for_weapon_profiles	~4262	M2M weapon_profiles_field
update_list_cache_for_weapon_accessories	~4273	M2M weapon_accessories_field
update_list_cache_for_upgrades	~4284	M2M upgrades_field

Phase 3: Remove Cache Infrastructure

Methods to delete from gyrinx/core/models/list.py:

• List.cost_cache_key() (~770)
• List.update_cost_cache() (~774)
• List.cost_int_cached property (~335) - replace usages with facts_with_fallback().wealth
• List.cost_int() method - verify no remaining callers first

Views to update:

• gyrinx/core/views/list.py - refresh_list_cost view (~847) - remove or update to use facts

Settings to potentially remove:

• CACHE_LIST_TTL if only used by this cache
• core_list_cache from CACHES config (if dedicated)

Tests to update:

• gyrinx/core/tests/test_models_core.py - remove cache assertions
• gyrinx/core/tests/test_list_refresh_cost.py - update or remove

Testing Strategy

1. After Phase 1: Verify can_use_facts() returns True for all list displays

   • Add temporary logging if needed
   • Check that cost_int_cached fallback is never hit

2. After Phase 2: Verify no errors, list costs still display correctly

   • All display methods should use facts() path
   • No performance regressions

3. After Phase 3: Verify all cost_display calls work without cache

   • Run full test suite
   • Manual smoke test on key pages

Execution Order

1. Phase 1: Fix prefetch gaps (can do now)
2. Run backfill in production
3. Verify all lists have actions via admin filter
4. Phase 2: Remove signals
5. Phase 3: Remove cache infrastructure

Notes

• credits_current is NOT a cache - it's the authoritative balance. Keep it.
• rating_current, stash_current, dirty are DB cache fields - KEEP them, they're central to facts system
• Only the in-memory Django cache (caches["core_list_cache"]) is being removed