Sqlalchemy duplicate entry when creating entries dynamically - flask

Question

In my application, I am using SQLAlchemy (via MySQL) with Flask, and in my current situation I need to create entries dynamically. For example, while creating an instance from table A, my script will see that we need to create a related entry B, so after creating B, it will attempt to assign B to one of the relationship attributes of the instance for A.
Here are some example models:
a_b = db.Table('a_b',
db.Column('a_id', db.Integer,
db.ForeignKey('a.id',
onupdate="CASCADE",
ondelete="CASCADE"),
primary_key=True),
db.Column('b_id', db.Integer,
db.ForeignKey('b.id',
onupdate="CASCADE",
ondelete="CASCADE"),
primary_key=True))
class ModelA(db.Model):
__tablename__ = 'a'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(100), nullable=False)
model_bs = db.relationship('ModelB', secondary=a_b,
back_populates='model_as')
class ModelB(db.Model):
__tablename__ = 'b'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(100), nullable=False)
model_as = db.relationship('ModelA', secondary=a_b,
back_populates='model_bs')
The problem I'm running into is an SQL duplicate entry exception, after running something like this:
from base import db
from base.database.models.models import ModelA, ModelB
def creation_test():
with db.session.no_autoflush:
# Start creating A
a = ModelA()
a.name = 'A Example'
# See that we need B, so create B
b = ModelB()
b.name = 'B Example'
b.model_as = []
db.session.add(b)
db.session.flush((b,))
# Use B in the relationship from A
a.model_bs = [b]
db.session.add(a)
db.session.flush((a,))
db.session.commit()
creation_test()
It will spit out an exception like:
sqlalchemy.exc.IntegrityError: (_mysql_exceptions.IntegrityError) (1062, "Duplicate entry '1-1' for key 'PRIMARY'") [SQL: u'INSERT INTO a_b (a_id, b_id) VALUES (%s, %s)'] [parameters: (1L, 1L)]
Is there something wrong with how the entries are being added to the session and/or how they are being flushed? Is it even possible to instantiate entries like this?
I appreciate any feedback.

Answer 1

After updating my version of SQLAlchemy and testing a few things, it seemed like I was able to solve this by removing the flushes in between creating each instance, and instead just perform a single flush before committing the session:
from base import db
from base.database.models.models import ModelA, ModelB
def creation_test():
with db.session.no_autoflush:
# Start creating A
a = ModelA()
a.name = 'A Example'
# See that we need B, so create B
b = ModelB()
b.name = 'B Example'
b.model_as = []
db.session.add(b)
# Use B in the relationship from A
a.model_bs = [b]
db.session.add(a)
db.session.flush()
db.session.commit()
creation_test()