Python Flask sqlalchemy Insert new column into table - heroku postgresql

BACKGROUND INFOS:

I have an app which is hosted by heroku on a postgresql DB.

I have already some data in this DB and now I have to add a new row in one of my tables.

Usually I deleted the old DB and recreated it. But in future if the project is live I will have to update tables without losing the data.

I could create a DUMP and delete the old database and recreate it as always. Then I could use a script and upload all existing data into the new DB. But this feels wrong.

WHAT I NEED:

In my current situation there is blog data table=blog on the database and I need to insert a new column into my table=zimmer so blog isnt even affected.

class Zimmer(Base):
    __tablename__ = 'zimmer'
    id = Column(Integer, primary_key=True)
    infofeld = Column(Text, nullable=False)
    land = Column(Text, nullable=False)
    bundesland = Column(Text, nullable=False)
    stadt = Column(Text, nullable=False)
    plz = Column(Text, nullable=False)
    strasse = Column(Text, nullable=False)
    hausnr = Column(Text, nullable=True)
    eigener_link = Column(Text, nullable=True)
    zimmer_lat = Column(Float, nullable=False)
    zimmer_lng = Column(Float, nullable=False)
    reingestellt_am = Column(Date, nullable=False)

This is the new value: eigener_link = Column(Text, nullable=True)

I am currently experimenting on localhost but so far I am only getting ProgrammingError because everytime I try to load a site where zimmer is shown it says there is no column eigener_link (that is logical).

WHAT I TRIED:

I tryed to try except the ProgrammingError in the line where it occured, which gave me an InternalError. Here I tryed to update the zimmer table and add the new column eigener_link:

try:
    for page in paginator:
        pages_list.append(page.number)
except ProgrammingError:
    update(Zimmer).values(eigener_link='Ihr Link')
    db_session.commit()

It gave me an InternalError. I checked the DB via pgAdmin and the new value has not been added.

    try:
        for page in paginator:
            pages_list.append(page.number)
    except ProgrammingError:
        db_session.execute('ALTER TABLE zimmer ADD eigener_link TEXT')
        db_session.commit()

This gave me also InternalError the transaction has been canceled.