FastAPI CRUD endpoints with Pydantic v2, Alembic and pytest tests

Prompt

Act as a senior backend developer specializing in Python, FastAPI and SQLAlchemy.

I want to add a complete module to an existing FastAPI API to manage {{resource}}.
- Fields and types: {{fields}}
- Business rules: {{rules}}
- Database: {{database}}
- Data access: {{data_access}}

Deliverables, in this order:
1. File layout (router, schemas, model, service or repository, tests) with one sentence justifying the split.
2. Separate Pydantic v2 schemas for create, partial update and response, with validation (`Field`, `field_validator`, `model_validator`) that enforces the business rules.
3. The database model and its Alembic migration, with indexes and unique constraints where they make sense.
4. A router with a paginated, filterable list, detail, create, partial update (PATCH) and delete. Use the correct status codes (201, 204, 404, 409, 422) and a `response_model` on every endpoint.
5. A database session injected with `Depends`, no global state, and clearly scoped transactions.
6. Tests with pytest and `httpx.AsyncClient` (or `TestClient` if data access is sync): happy path, validation, missing resource and business-rule conflict, against an isolated test database.
7. Commands to run the app, apply the migration and run the tests.

Constraints:
- Python 3.11+, fully typed, no unnecessary dependencies.
- Do not invent fields or rules: if anything is ambiguous, ask me before writing code or mark it as [FILL IN].
- Point out which decisions depend on how the rest of my project is set up (auth, settings, table naming).

Format: one code block per file with its path as the heading, then a list of assumptions at the end.

How to use it

Usage example

Sample output

from datetime import datetime, timedelta, timezone
from enum import Enum
from pydantic import BaseModel, Field, field_validator

class BookingStatus(str, Enum):
    pending = "pending"
    confirmed = "confirmed"
    cancelled = "cancelled"

class BookingCreate(BaseModel):
    guest_name: str = Field(min_length=1, max_length=120)
    phone: str = Field(pattern=r"^\+?[0-9 ]{7,20}$")
    starts_at: datetime
    party_size: int = Field(ge=1, le=12)

    @field_validator("starts_at")
    @classmethod
    def within_booking_window(cls, v: datetime) -> datetime:
        if v.tzinfo is None:
            raise ValueError("starts_at must include a timezone")
        now = datetime.now(timezone.utc)
        if v < now or v > now + timedelta(days=60):
            raise ValueError("bookings must be between now and 60 days ahead")
        return v

class BookingUpdate(BaseModel):
    starts_at: datetime | None = None
    party_size: int | None = Field(default=None, ge=1, le=12)
    status: BookingStatus | None = None

Key facts

Category

Programming & development · Python

Output type

Code

Level

Intermediate

Designed for

ChatGPT, Claude, Gemini and Microsoft Copilot

Verified in

Pending editorial verification

Variables

Resource, Fields and types, Business rules, Database and Data access

Language

English

Price

Free, no sign-up

Updated

Nov 7, 2026

Frequently asked questions