Example of flask with sqlalchemy without flask_sqlalchemy for https://www.reddit.com/r/flask/comments/8n65hy/flasksqlalchemy_without_using_flask_sqlalchemy/

__init__.py

from flask import Flask, request

from .database import session, init_db, teardown_session
from .models import User

app = Flask(__name__)
app.config.from_pyfile('config.py')

init_db(app)


@app.route('/')
def dashboard():
    return ', '.join(user.email for user in User.query)

@app.route('/user/create', methods=['POST'])
def user_create():
    if 'email' not in request.json:
        return "'email' is required", 422

    email = request.json['email']
    user = User.query.filter(User.email.ilike(email.strip())).first()

    if user:
        return f"User with email '{email}' already exists", 409

    user = User(email=email.strip())
    session.add(user)
    session.commit()

    return user.email, 201

config.py

DATABASE_URI = 'sqlite:///test.db'

database.py

from flask import current_app
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

Session = sessionmaker(autocommit=False,
                       autoflush=False)
session = scoped_session(Session)

Base = declarative_base()
Base.query = session.query_property()


from . import models


def init_db(app=current_app):
    engine = create_engine(app.config['DATABASE_URI'], convert_unicode=True)

    Session.configure(bind=engine)
    Base.metadata.create_all(bind=engine)

    app.teardown_request(teardown_session)

def teardown_session(exception=None):
    session.remove()

models.py

from sqlalchemy import Column, Integer, String
from .database import Base


class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, autoincrement=True, primary_key=True)
    email = Column(String(255), unique=True)

Pipfile

[[source]]
url = "https://pypi.python.org/simple"
verify_ssl = true
name = "pypi"

[packages]
flask = "*"
sqlalchemy = "*"

[dev-packages]

[requires]
python_version = "3.6"