flask sqlalchemy unique constraint on multiple columns [duplicate]

This question already has an answer here:

• sqlalchemy unique across multiple columns 1 answer

I am using SQLite and Flask on my project, i want apply unique constraint on multiple columns, i used UniqueConstraint as mentioned on this answer, but db still allows entering duplicate records. What am i doing wrong or how can i add unique contraint on multiple columns on sqlite?

from sqlalchemy import UniqueConstraint

class TeacherGroup(db.Model):
    __tablename__ = 'tbl_my_group'
    id = db.Column(GUID, primary_key=True)
    col1 = db.Column(db.String(80),
                     db.ForeignKey(tbl1.__tablename__ + '.id'),
                     nullable=False)
    col2 = db.Column(GUID,
                     db.ForeignKey(tbl2.__tablename__ + '.id'),
                     nullable=False)
    db.UniqueConstraint('col1', 'col1', name='col1_col2')