[Sqlalchemy-tickets] Issue #4055: merge does not follow primary key with default value (zzzeek/sqlalchemy)

[pkyosx <iss...@bi...>]

New issue 4055: merge does not follow primary key with default value
https://bitbucket.org/zzzeek/sqlalchemy/issues/4055/merge-does-not-follow-primary-key-with

pkyosx:

merge will try to select first using primary key; however, a primary key with default value does not take effect on merge.

I have run the following example under sqlalchemy-1.1.13.

```
#!python

from sqlalchemy import create_engine, MetaData
from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

Base = declarative_base()

engine = create_engine('sqlite:///:memory:', echo=True)

class User(Base):
	__tablename__ = 'users'
	family_id = Column(Integer, primary_key=True, autoincrement=False)
	member_id = Column(Integer, primary_key=True, autoincrement=False, default=1)
	name = Column(String)

Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine)

session = Session()

john = User(family_id=1, name="john")
peter = User(family_id=1, name="peter")

session.merge(john)
session.commit()

session.merge(peter)

# a select does not issued for peter because missing a member_id in constructor
# however, member_id has default value setting...
# it supposed to take default value into account.

# A error like the following will happen.
# sqlalchemy.exc.IntegrityError: (sqlite3.IntegrityError) UNIQUE constraint failed: users.family_id, users.member_id [SQL: u'INSERT INTO users (family_id, member_id, name) VALUES (?, ?, ?)'] [parameters: (1, 1, 'peter')]
session.commit()
```