City model - Making a unique record based on city name, state, and country (3 fields combined)

[itsahsiao]

Re: Issue #3 for data modeling, City model right now is simple:

class City(db.Model):
    """City where the restaurant is in."""

    __tablename__ = "cities"

    city_id = db.Column(db.Integer, autoincrement=True, primary_key=True)
    city_name = db.Column(db.String(100), nullable=False)
    updated_At = db.Column(db.DateTime, default=datetime.datetime.utcnow)

However, there could be two cities with the same name, but in different states or countries.
Per mentor, need to make it unique across 3 fields (city, state, country), so that there will not be a duplicated record in the table.

Something to look into: http://stackoverflow.com/questions/10059345/sqlalchemy-unique-across-multiple-columns