Aller au contenu

WAC & Cost Basis

WAC (Weighted Average Cost) — also called PMC (Prezzo Medio di Carico) — is LibreFolio's primary cost basis methodology. The implementation lives in backend/app/services/wac_service.py and backend/app/utils/financial_utils.py.

Architecture: Two Layers

The computation is split into two layers:

compute_wac_iterative()          ← async DB layer
    └─ compute_wac_from_txlist() ← pure math layer (no DB, no IO)
Layer File Responsibility
compute_wac_iterative wac_service.py Queries DB, resolves FX, builds input list
compute_wac_from_txlist financial_utils.py Pure iterative WAC math, no side effects

This separation makes the pure math layer fully unit-testable without a database.

compute_wac_iterative() — Async Preparation Layer

async def compute_wac_iterative(
    session: AsyncSession,
    broker_id: int,
    asset_id: int,
    as_of_date: date,
    asset_currency: str,
    excluded_tx_ids: list[int] | None = None,
) -> WACPreviewResultItem:

Steps performed:

  1. Query all transactions for (broker_id, asset_id) with date ≤ as_of_date and quantity ≠ 0
  2. Exclude any IDs in excluded_tx_ids (used for WAC preview of a not-yet-saved transaction)
  3. Determine target currency: inferred from the most frequent acquisition currency; falls back to asset_currency
  4. FX conversion: for acquisitions in a different currency, fetch FX rates and convert to the target currency via convert_bulk()
  5. Delegate to compute_wac_from_txlist() for the iterative calculation

WAC Algorithm

The WAC algorithm maintains a running inventory of (quantity, average cost):

For each transaction in chronological order:
  BUY:       new_wac = (old_qty × old_wac + new_qty × unit_cost) / (old_qty + new_qty)
             inventory += new_qty
  SELL:      inventory -= sold_qty   (WAC does not change on a sell)
  TRANSFER+: treated like BUY using cost_basis_override as unit cost
  TRANSFER-: treated like SELL
  ADJUSTMENT+: treated like BUY

The algorithm handles zero-crossing (position going to zero): when inventory reaches exactly zero, the WAC is reset to zero. A subsequent acquisition starts a fresh average.

Cost Basis Override (Manual WAC)

Each transaction has two optional fields for manual cost basis override:

Field Description
cost_basis_override Amount in cost_basis_currency representing the total acquisition cost
cost_basis_currency Currency of the override (may differ from the transaction currency)

When cost_basis_override is set, the WAC engine uses it instead of computing from the transaction amount. This is essential for:

  • TRANSFER incoming leg — the receiving broker does not know the original purchase price; the user sets it manually
  • ADJUSTMENT+ — arbitrary quantity adjustments require an explicit cost basis

If cost_basis_override is None on a BUY, the engine uses amount (gross transaction value).

WAC Preview (Real-time)

The transaction form displays a WAC preview before the transaction is saved:

GET /transactions/wac-preview?broker_id=…&asset_id=…&date=…&…

The preview calls compute_wac_iterative() with the pending transaction included in the input list (injected as an in-memory row, not persisted) to show the projected cost basis after save.

Usage in execute_batch()

After creating/updating transactions in a batch, TransactionService._compute_wac_for_auto_items() runs WAC for each affected (broker_id, asset_id) pair where the transaction has cost_basis_mode = "auto":

await self._compute_wac_for_auto_items(batch_results, session)

The result is written back to transaction.cost_basis_override so the DB always stores the resolved cost basis (no re-computation on read).

Data Structures

@dataclass
class WACInputTX:
    tx_id: int
    type: str               # "BUY", "SELL", "TRANSFER", "ADJUSTMENT"
    date: date
    quantity: Decimal
    unit_cost_converted: Decimal | None   # in target currency (post FX-conversion)
    original_currency: str
    is_pending: bool        # True for preview-only rows

@dataclass
class WACPreviewResultItem:
    wac: Currency           # Current WAC after all transactions
    wac_qualifying_txs: list[WACQualifyingTX]   # Which transactions affected WAC
    wac_missing_pairs: list[int]                 # IDs of transactions with no cost basis