How to add a foreign key to an existing table in SQLAlchemy?

Question

Python

There are such classes where several indicators (Indicator) belong to one report (Report):

class Report(Base):
    __tablename__ = 'report'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    comment = Column(String)
    regularity = Column(Integer)
    theme = Column(Integer)


class Indicator(Base):
    __tablename__ = 'indicator'
    date = Column(String, primary_key=True)
    value = Column(String)
    report = Column(Integer, primary_key=True)

The database already has data
The report field in the Indicator table contains Report.id
The date, report fields in the Indicator table are a composite primary key
How to add a many-to-one relationship (many indicators to one report) if the database already exists and it already has data?

Answer 1

Migrations
https://pypi.python.org/pypi/sqlalchemy-migrate

Answer 2

There is such a "Mega tutorial Flask", it was translated on Habré, there is an article about databases with an example of sqlalchemy migration (sqlite)