RuntimeError：未找到应用程序。 在视图 function 内工作或推送应用程序上下文。 FLASK SQLAlchemy 错误

Question

application.py

from flask import Flask, render_template
from wtform_fields import *
from models import *
app = Flask(__name__)
app.secret_key='REPLACE LATER'
app.config['SQLALCHEMY_DATABASE_URI']='*db link*'
db = SQLAlchemy(app)
@app.route("/", methods=['GET', 'POST'])
def index():

    reg_form = RegistrationForm()

    # Update database if validation success
    if reg_form.validate_on_submit():
        username = reg_form.username.data
        password = reg_form.password.data
        user_object = User.query.filter_by(username=username).first()
        if user_object:
         return "Already Taken"
        user =User(username=username,password=password)
        db.session.add(user)
        db.session.commit()
        return "Inserted into DB"
    return render_template("index.html", form=reg_form)
if __name__ == "__main__":
    app.run(debug=True)

models.py

from flask_sqlalchemy import SQLAlchemy 
db = SQLAlchemy()
class User(db.Model):
    """ User model """
    __tablename__="users"
    id=db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(25) , unique=True, nullable=False)
    password = db.Column(db.String(), nullable=False)
    db.create_all()

I am getting this error:

D:\PostgreSQL\12\bin\GSTChat\venv\lib\site-packages\flask_sqlalchemy\__init__.py", line 1094, in create_all
    self._execute_for_all_tables(app, bind, 'create_all')
  File "D:\PostgreSQL\12\bin\GSTChat\venv\lib\site-packages\flask_sqlalchemy\__init__.py", line 1071, in _execute_for_all_tables
    app = self.get_app(app)
  File "D:\PostgreSQL\12\bin\GSTChat\venv\lib\site-packages\flask_sqlalchemy\__init__.py", line 1042, in get_app
    raise RuntimeError(
**RuntimeError: No application found. Either work inside a view function or push an application context. See http://flask-sqlalchemy.pocoo.org/contexts/.**

I even looked for solution in the documentation but, I am not able to resolve the error.

Answer 1

EDIT: Adding another answer...

You can have it as you already do in the models.py file, and initialize it later in your application file:

application.py

from flask import Flask, render_template
from wtform_fields import *
from models import *

app = Flask(__name__)
# Other code you have
db.init_app(app)

Answer 2: I see you are defining db in your application.py file, and again in your models.py file. You should import it from your app in your models file.

models.py

from flask_sqlalchemy import SQLAlchemy
from app import db

class User(db.Model):
    """ User model """
    __tablename__="users"
    id=db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(25) , unique=True, nullable=False)
    password = db.Column(db.String(), nullable=False)

I hope this is helpful and solves your error: :) You may also want to put the db.create_all() in your application.py file (after importing models), instead of in a model itself. Cheers.