Add .env-example file and update configuration for environment variables

- Created .env-example for environment variable setup
- Updated README to reflect changes in .env file creation
- Modified config.py to include new environment variables and DATABASE_URL property
- Enhanced database.py with connect and disconnect functions
- Updated main.py to manage database connection lifecycle
- Adjusted compose.yaml for consistent environment variable usage

app/config.py

@@ -1,6 +1,23 @@
 from pydantic_settings import BaseSettings
+from pydantic import computed_field
 
 class Settings(BaseSettings):
-    DATABASE_URL: str = "${DATABASE_URL}"
+    POSTGRES_USER: str
+    POSTGRES_PASSWORD: str
+    POSTGRES_DB: str
+    API_PORT: int = 8000
+    DEBUG: bool = False
+
+    @computed_field
+    @property
+    def DATABASE_URL(self) -> str:
+        return (
+            f"postgresql+asyncpg://"
+            f"{self.POSTGRES_USER}:{self.POSTGRES_PASSWORD}"
+            f"@db/{self.POSTGRES_DB}"
+        )
+
+    class Config:
+        env_file = ".env"
 
 settings = Settings()

app/database.py

@@ -1,11 +1,22 @@
+from typing import AsyncGenerator
 from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
 from sqlalchemy.orm import sessionmaker
-from sqlalchemy import text
 from app.config import settings
 
-engine = create_async_engine(settings.DATABASE_URL, echo=True)
+engine = create_async_engine(settings.DATABASE_URL, echo=settings.DEBUG)
 AsyncSessionLocal = sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
 
-async def get_db():
+async def connect():
+    pass  # engine initializes at module level
+
+async def disconnect():
+    await engine.dispose()
+
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
     async with AsyncSessionLocal() as session:
-        yield session
+        try:
+            yield session
+            await session.commit()
+        except Exception:
+            await session.rollback()
+            raise

app/main.py

@@ -1,14 +1,15 @@
 from contextlib import asynccontextmanager
 from fastapi import FastAPI
-from app.database import engine
+from app.database import connect, disconnect
 from app.routers import authors, books
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    yield # tables already created by Create.sql
-    await engine.dispose()
+    await connect()
+    yield
+    await disconnect()
 
 app = FastAPI(title="Database API", lifespan=lifespan)
 
-app.include_router(authors.router)
-app.include_router(books.router)
+for router in [authors.router, books.router]:
+    app.include_router(router)