Flask ' application not registered on db' - flask-sqlalchemy

Question

I tried to check answers to other questions with the same error. But the situation of those seem different from mine.
I can use Role.query.all() in .\xxxapp\admin\views.py, but when I call Role.query.all() in .\xxxapp\admin\forms.py, I got this:
raise RuntimeError('application not registered on db '
RuntimeError: application not registered on db instance and no application bound to current context
My folders are like this:
.\
|-xxxapp\
| |-admin\
| | |__init__.py
| | |forms.py
| | |views.py
| |-user\
| | |__init__.py
| | |forms.py
| | |models.py
| | |views.py
| |....
| |-extensions.py
| |-app.py
|manage.py
app is created in .\xxxapp\extensions.py by a app creator function 'create_app', and db is created in .\xxxapp\extensions.py. All initiators such as db.init(app) are called in 'create_app'. While 'create_app' is called in manage.py. I don't understand why in views.py app and db know each other while in forms.py they don't.
Now here is my code in .\xxxapp\admin\views.py
from flask.ext.wtf import Form
from xxxapp.user.models import User, Role
class UserForm(Form):
roles = Role.query.all() # this doesn't work
...
and .\xxxapp\admin\models.py
from flask import (current_app, Blueprint, request, render_template, flash, url_for, redirect, session)
from flask.ext.user import current_user, login_required, roles_required
from xxxapp.extensions import db, user_manager
from xxxapp.user.models import User, Role
blueprint = Blueprint('admin', __name__, static_folder="../static")
#blueprint.route('/user/<int:user_id>', methods=['GET', 'POST'])
#login_required
#roles_required('admin')
def user(user_id):
roles = Role.query.all() #this works
Class Role is defined in .\xxxapp\user\models.py:
from xxxapp.extensions import db
.....
class Role(db.Model):
id = db.Column(db.Integer(), primary_key=True)
name = db.Column(db.String(50), unique=True)
description = db.Column(db.String(255))
app factory in app.py contains:
...(imported modules are omitted)
...
__all__ = ['create_app']
DEFAULT_BLUEPRINTS = (frontend,d5user,api,admin)
def create_app(config=None, app_name=None, blueprints=None):
if app_name is None:
app_name = DefaultConfig.PROJECT
if blueprints is None:
blueprints = DEFAULT_BLUEPRINTS
app = Flask(import_name=app_name, instance_path=INSTANCE_FOLDER_PATH, instance_relative_config=True)
app.config.from_object(DefaultConfig)
mail = Mail(app)
init_error_logger_with_email_handler(app)
register_extensions(app)
register_blueprints(app, blueprints)
register_errorhandlers(app)
configure_template_filters(app)
return app
def register_blueprints(app, blueprints):
for blt in blueprints:
app.register_blueprint(blt)
def register_extensions(app):
assets.init_app(app)
bcrypt.init_app(app)
cache.init_app(app)
db.init_app(app)
debug_toolbar.init_app(app)
migrate.init_app(app, db)
user_manager.init_app(app) # Init Flask-User and bind to app
return None
... more registration function are omitted.
I must have missed something, but have no clue where. Maybe it is an easy problem for experienced. Any suggestion will be appreciated.