[Sqlalchemy-tickets] Issue #4278: NULL does not override default value if new instance is created when session.merge(row) (zzzeek/sqlalchemy)

[jan.budinsky <iss...@bi...>]

New issue 4278: NULL does not override default value if new instance is created when session.merge(row)
https://bitbucket.org/zzzeek/sqlalchemy/issues/4278/null-does-not-override-default-value-if

jan.budinsky:

Default column value is not overridden by `NULL` in `Declarative API` as it should be.

If a column has a non-null default value that is overridden by another non-null value, everything works as expected - the specified value is always inserted as the value of the column instead of the default one.

This however does not seem to be the case when overriding non-null default value with `NULL`. If a matching row (via primary key) has been already created (`merge` does `UPDATE`), the column will have `NULL` filled as its value, as expected. But if a matching row hasn't been created yet (`merge` does `INSERT INTO`), the column will have default value instead of expected `NULL`.

The issue is the same using both `postgresql` and `in-memory sqlite`.

Please see the script below reproducing the issue, thank you.
```
#!python

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, scoped_session
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column
from sqlalchemy.dialects.postgresql import BIGINT, VARCHAR


CONN_STR = "sqlite:///:memory:"
Base = declarative_base()


class Table(Base):
    __tablename__ = "table"

    id = Column(BIGINT, primary_key=True)
    field = Column(VARCHAR, default="placeholder")


def merge(session, row):
    session.begin()
    try:
        session.merge(row)
        session.commit()
    except:
        session.rollback()
        raise


def select(session, table, **criteria):
    return session.query(table).filter_by(**criteria).all()


def delete(session, table, **criteria):
    return session.query(table).filter_by(**criteria).delete()


def main():
    engine = create_engine(CONN_STR, echo=False)
    Base.metadata.create_all(engine)  # create table
    session = scoped_session(sessionmaker(bind=engine, autocommit=True))()

    data_null = {
        "id": 1,
        "field": None
    }
    data_value = {
        "id": 2,
        "field": "value"
    }

    # cleanup
    delete(session=session, table=Table, id=data_null["id"])
    delete(session=session, table=Table, id=data_value["id"])

    merge(session=session, row=Table(**data_null))
    print(select(session=session, table=Table, id=data_null["id"])[0].field)  # "placeholder" ???
    merge(session=session, row=Table(**data_null))
    print(select(session=session, table=Table, id=data_null["id"])[0].field)  # None

    merge(session=session, row=Table(**data_value))
    print(select(session=session, table=Table, id=data_value["id"])[0].field)  # "value"
    merge(session=session, row=Table(**data_value))
    print(select(session=session, table=Table, id=data_value["id"])[0].field)  # "value"


if __name__ == "__main__":
    main()
```
Environment:
```
python version - 3.6.3
SQLAlchemy version - 1.2.8
```