[Sqlalchemy-tickets] Issue #3794: Setting column as None does not explicitly add the NULL to insert statement (zzzeek/sqlalchemy)

[Konsta V. <iss...@bi...>]

New issue 3794: Setting column as None does not explicitly add the NULL to insert statement
https://bitbucket.org/zzzeek/sqlalchemy/issues/3794/setting-column-as-none-does-not-explicitly

Konsta Vesterinen:

The following code illustrates this problem:
```
#!python

import sqlalchemy as sa
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base


engine = sa.create_engine(
    'postgres://postgres@localhost/test'
)
engine.echo = True
Base = declarative_base()
Session = sessionmaker(bind=engine)
session = Session()


class Question(Base):
    __tablename__ = 'question'
    id = sa.Column(sa.Integer, primary_key=True)

    weight = sa.Column(sa.Integer, default=1)


Base.metadata.create_all(bind=session.bind)


q = Question(weight=None)
session.add(q)
session.commit()

assert q.weight is None
```

The problem also exists by changing the ``default=1`` to ``server_default='1'``. Also in this case the explicit weight=NULL is not added to INSERT statement.