Can SQLAlchemy's session.merge() update its result with newer data from the database?

Question

The SQLAlchemy documentation says \"session.merge() reconciles the current state of an instance and its associated children with existing data in the database\".

Does th

Answer 1

One thing I noticed about merge is that even accessing a field on the unattached object will cause it to be modified during a merge.

For example, if I have a simple class

class X(Base):
    __tablename__= 'x'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    value = Column(String)

and a row

(1, 'foo', 'bar')

then the following seems to merge nicely:

x = X(id=1)
merged_x = session.merge(x)

print merged_x.name         # 'foo'
print merged_x.description  # 'bar'

But if I even read name or description, this happens

x = X(id=1)
print x.name                # None

merged_x = session.merge(x)

print merged_x.name         # None
print merged_x.description  # 'bar'

This is counterintuitive and, I'd argue, incorrect. Is there a way to turn off this behavior? Apparently the mere presence of an attribute in __dict__ causes this to happen. This 'feature' should be noted in the documentation.