Session: scaffold-invoice-service — 2026-08-18

Project: ledger-api · Branch: main · Mode: default · Model: claude-sonnet-4-6

Summary

Bootstrapped the invoicing service: FastAPI app factory, SQLAlchemy 2.0 models for customers, invoices and line items, Alembic baseline migration, and a pytest fixture that spins up Postgres in Docker. Money is stored as integer minor units from day one.

Conversation

Turn 1 — User

Scaffold a FastAPI service for invoices. Postgres, SQLAlchemy 2.0, Alembic. Money must never be a float.

Turn 1 — Assistant

Plan: app factory in ledger/app.py, typed models in ledger/models.py, amounts as BIGINT minor units plus an ISO-4217 currency column, Alembic baseline, Docker-based Postgres fixture for tests.

class LineItem(Base):
    __tablename__ = "line_items"

    id: Mapped[int] = mapped_column(primary_key=True)
    invoice_id: Mapped[int] = mapped_column(ForeignKey("invoices.id"))
    description: Mapped[str] = mapped_column(String(200))
    quantity: Mapped[int] = mapped_column(default=1)
    unit_amount: Mapped[int]  # minor units (cents)
    currency: Mapped[str] = mapped_column(String(3))

Tools used in this turn:

• Write: ledger/app.py
• Write: ledger/models.py
• Bash: alembic init migrations — exit 0

Turn 2 — User

Add the line-item total as a computed column.

Turn 2 — Assistant

Done as a SQLAlchemy column_property summing quantity * unit_amount; invoice totals are derived, never stored, so they cannot drift. 9 tests pass.

Tools used in this turn:

• Edit: ledger/models.py — column_property for totals
• Bash: pytest -q — 9 passed

Connections

• [[ledger-api]]
• [[FastAPI]]
• [[SQLAlchemy]]
• [[Alembic]]
• [[MoneyAsMinorUnits]]