SqlAlchemy mapper mapped class falied to locate

I have two tables one to many relationship. I am using flask_sqlalchemy. The parent table is being created but can't child table got an error. I am having init.py in my models folder.

Parent Table:

class Chalans(db.Model):
__tablename__ = "chalans"

create_time = db.Column(db.DateTime)
chalan_id = db.Column(db.String(20), primary_key=True, nullable=False)
customer_id = db.Column(db.String(20))
.....
chalan_details = db.relationship("ChalanDetails", backref="chalans", lazy="dynamic")

Child Table:

class ChalanDetails(db.Model):
__tablename__ = "chalan_details"

create_time = db.Column(db.DateTime)
chalan_id = db.Column(db.String(20), db.ForeignKey("chalans.chalan_id"), primary_key=True,   nullable=False)
chalan_row = db.Column(db.Integer, primary_key=True, nullable=False)
....
remarks = db.Column(db.String(200))

I am having this error below:

sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class Chalans->chalans, expression 'ChalanDetails' failed to locate a name ('ChalanDetails'). If this is a class name, consider adding this relationship() to the <class 'models.chalans.Chalans'> class after both dependent classes have been defined.