Let's say something like this is being returned.

RuntimeError: The current Flask app is not registered with this 'SQLAlchemy' instance. Did you forget to call 'init_app', or did you create multiple 'SQLAlchemy' instances?

And let's say this occurs when attempting to append records to a table using Flask. In this example, perhaps your view has something like this.

from flask import Blueprint, render_template
from sqlalchemy import func, exc
from . import db
from .models import users

views = Blueprint('views', __name__)

@views.route('/test')
def test():

  firstname = "john"
  lastname  = "doe"

  try:
      db.session.add(data)
  except Exception as exception:
      print("got the following exception when attempting db.session.add(data))
      print("data = " + str(data))
      print(exception)
  finally:
      db.session.close()

  try:
      db.session.commit()
  except exc.SQLAlchemyError as sqlalchemyerror:
      print("got the following SQLAlchemyError: " + str(sqlalchemyerror))
  except Exception as exception:
      print("got the following Exception: " + str(exception))
  finally:
      db.session.close()

  return render_template('test.html')

Notice in this example that "firstname" and "lastname" are being appended to the users table. In this scenario, your model should have something like this.

from . import db

class users(db.Model):
  id           = db.Column(db.Integer,     nullable=False, unique=True, primary_key=True)
  firstname    = db.Column(db.String(50), nullable=False, unique=True)
  lastname     = db.Column(db.String(50), nullable=False, unique=False)

Notice the error says "Did you forget to call init_app". Ensure __init__.py includes init_app.

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

def app_obj():
    app = Flask(__name__)

    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///example.db'
    db.init_app(app)

    # this is needed in order for database session calls (e.g. db.session.commit)
    with app.app_context():
      try:
          db.create_all()
      except Exception as exception:
          print("got the following exception when attempting db.create_all() in __init__.py: " + str(exception))
      finally:
          print("db.create_all() in __init__.py was successfull - no exceptions were raised")
        from .models import users

    return app

If issues persist, add the following to __init__.py.

app.config["SQLALCHEMY_ECHO"] = True
app.config["SQLALCHEMY_RECORD_QUERIES"] = True