How to build a flask application around an already existing database?

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I’d say your question has nothing to do with flask at all. For example, you don’t have a problem with the templates, routes, views or logon decorators.

Where you struggle at is at SQLAlchemy.

So my suggestion is to ignore Flask for a while and get used to SQLAlchemy first. You need to get used to your existing database and how to access it from SQLAlchemy. Use some MySQL documentation tool to find your way around this. The start with something like this (note that it has nothing to do with Flask ask all … yet):

#!/usr/bin/python
# -*- mode: python -*-

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite:///webmgmt.db', convert_unicode=True, echo=False)
Base = declarative_base()
Base.metadata.reflect(engine)


from sqlalchemy.orm import relationship, backref

class Users(Base):
    __table__ = Base.metadata.tables['users']


if __name__ == '__main__':
    from sqlalchemy.orm import scoped_session, sessionmaker, Query
    db_session = scoped_session(sessionmaker(bind=engine))
    for item in db_session.query(Users.id, Users.name):
        print item

In the line “engine =” you need to provide your path to your MySQL database, so that SQLAlchemy finds it. In my case I used a pre-existing sqlite3 database.

In the line “class Users(Base)” you need to use one of existing tables in your MySQL database. I knew that my sqlite3 database had a table named “users”.

After this point, SQLalchemy knows how to connect to your MySQL database and it knows about one of the tables. You need now to add all the other tables that you care for. Finally, you need to specify relationships to SQLalchemy. Here I mean things like one-to-one, one-to-many, many-to-many, parent-child and so on. The SQLAlchemy web site contains a rather lenghty section about this.

After the line “if __name__ == '__main__'” just comes some test code. It will be executed if I don’t import my python script, but run. Here you see that I create a DB session and is that for a very simply query.

My suggestion is that you first read about the important parts of SQLAlchemy’s documentation, for example the descriptive table definition, the relationship model and how to query. Once you know this, you can change the last part of my example into a controller (e.g. using Python’s yield method) and write a view that uses that controller.

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