python - unable to create autoincrementing primary key with flask-sqlalchemy

Question

I want my model's primary key to be an autoincrementing integer. Here is how my model looks like

class Region(db.Model):
    __tablename__ = 'regions'
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    name = db.Column(db.String(100))
    parent_id = db.Column(db.Integer, db.ForeignKey('regions.id'))
    parent = db.relationship('Region', remote_side=id, primaryjoin=('Region.parent_id==Region.id'), backref='sub-regions')
    created_at = db.Column(db.DateTime, default=db.func.now())
    deleted_at = db.Column(db.DateTime)

The above code creates my table but does not make id autoincrementing. So if in my insert query I miss the id field it gives me this error

ERROR: null value in column "id" violates not-null constraint

So I changed the id declaration to look like this

id = db.Column(db.Integer, db.Sequence('seq_reg_id', start=1, increment=1),
               primary_key=True)

Still the same error. What is wrong with the code above?

Answer 1

Nothing is wrong with the above code. In fact, you don't even need autoincrement=True or db.Sequence('seq_reg_id', start=1, increment=1), as SQLAlchemy will automatically set the first Integer PK column that's not marked as a FK as autoincrement=True.

Here, I've put together a working setup based on yours. SQLAlechemy's ORM will take care of generating id's and populating objects with them if you use the Declarative Base based class that you've defined to create instances of your object.

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.debug = True
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql://user:password@localhost/testdb'
app.config['SQLALCHEMY_ECHO'] = True
db = SQLAlchemy(app)

class Region(db.Model):
    __tablename__ = 'regions'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100))

db.drop_all()
db.create_all()

region = Region(name='Over Yonder Thar')
app.logger.info(region.id) # currently None, before persistence

db.session.add(region)
db.session.commit()
app.logger.info(region.id) # gets assigned an id of 1 after being persisted

region2 = Region(name='Yet Another Up Yar')
db.session.add(region2)
db.session.commit()
app.logger.info(region2.id) # and 2

if __name__ == '__main__':
    app.run(port=9001)