How to ensure data uniqueness in SQLAlchemy?

Question

SQL

There is a table:

PostVoice(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'),nullable=False)
    blogpost_id = db.Column(db.Integer, db.ForeignKey('blog_post.id'),nullable=False)
    value = db.Column(db.Enum(VoiceEnum),nullable=False)

How to ensure that a user can only vote once. That is, if the user has already gone for this post, his voice was changed, and another record of his voice was not created.