from typing import List
from fastapi import APIRouter, Depends, HTTPException
from sqlalchemy.orm import Session

from app.db import get_db
from app.models.user import User as UserModel
from app.schemas.user import User, UserCreate, UserUpdate

router = APIRouter()

@router.post("/users", response_model=User, status_code=201)
def create_user(user: UserCreate, db: Session = Depends(get_db)):
    db_user = UserModel(**user.dict())
    db.add(db_user)
    db.commit()
    db.refresh(db_user)
    return db_user

@router.get("/users", response_model=List[User])
def get_users(db: Session = Depends(get_db)):
    users = db.query(UserModel).all()
    return users

@router.get("/users/{user_id}", response_model=User)
def get_user(user_id: int, db: Session = Depends(get_db)):
    db_user = db.query(UserModel).filter(UserModel.id == user_id).first()
    if not db_user:
        raise HTTPException(status_code=404, detail="User not found")
    return db_user

@router.put("/users/{user_id}", response_model=User)
def update_user(user_id: int, user: UserUpdate, db: Session = Depends(get_db)):
    db_user = db.query(UserModel).filter(UserModel.id == user_id).first()
    if not db_user:
        raise HTTPException(status_code=404, detail="User not found")
    update_data = user.dict(exclude_unset=True)
    for key, value in update_data.items():
        setattr(db_user, key, value)
    db.commit()
    db.refresh(db_user)
    return db_user

@router.delete("/users/{user_id}", status_code=204)
def delete_user(user_id: int, db: Session = Depends(get_db)):
    db_user = db.query(UserModel).filter(UserModel.id == user_id).first()
    if not db_user:
        raise HTTPException(status_code=404, detail="User not found")
    db.delete(db_user)
    db.commit()
    return {"message": "User deleted successfully"}
```


The endpoints are:

1. `POST /users`: Creates a new user based on the provided `UserCreate` model data.
2. `GET /users`: Retrieves a list of all users.
3. `GET /users/{user_id}`: Retrieves a specific user by ID.
4. `PUT /users/{user_id}`: Updates an existing user with the provided `UserUpdate` model data.
5. `DELETE /users/{user_id}`: Deletes a user by ID.

The code uses dependency injection to obtain a SQLAlchemy database session from the `get_db` function (assumed to be defined elsewhere). It also handles HTTP exceptions for cases where a user is not found (404 Not Found).

Note that this is a basic implementation, and you may need to add additional functionality, such as authentication, authorization, and input validation, depending on your application's requirements.