[Sqlalchemy-tickets] Issue #3522: Attribute history not tracked when using session.merge() (zzzeek/sqlalchemy)

[Sylwester K. <iss...@bi...>]

New issue 3522: Attribute history not tracked when using session.merge()
https://bitbucket.org/zzzeek/sqlalchemy/issues/3522/attribute-history-not-tracked-when-using

Sylwester Kardziejonek:

I've noticed that the attribute history of my objects is not tracked.
The object comes from session.merge() like it was just freshly retrieved from database.


```
#!python
shipping_id = int(request.matchdict.get('id'))

# Get model from db
db_shipping = db.session.query(m.Shipping).get(shipping_id)

# Create updated instance
updated_shipping = m.Shipping(id=shipping_id, price=999)

# Merge it with session
merged = db.session.merge(updated_shipping)
# I've noticed an actual UPDATE is emitted at this point

# Attributes values are correctly reflected, but the history is all 'unchanged'

# Simply iterates over history of each attribute
changeset(merged)

# History is all 'unchanged'
```

>From the other hand, if I add one line, this works:

```
#!python
shipping_id = int(request.matchdict.get('id'))

# Get model from db
db_shipping = db.session.query(m.Shipping).get(shipping_id)

# When I add this, the history is tracked properly
changeset(db_shipping)

# Create updated instance
updated_shipping = m.Shipping(id=shipping_id, price=999)

# Merge it with session
merged = db.session.merge(updated_shipping)

# Attributes values are correctly reflected, history state is correct

# Simply iterates over history of each attribute
changeset(merged)

```


```
#!python
def changeset(obj):
    data = {}
    for prop in obj.__mapper__.iterate_properties:
        history = get_history(obj, prop.key)
        if history.has_changes():
            old_value = history.deleted[0] if history.deleted else None
            new_value = history.added[0] if history.added else None
            if new_value:
                data[prop.key] = [new_value, old_value]
    return data

```