[Sqlalchemy-tickets] Issue #3387: add_entity and before_compile event (zzzeek/sqlalchemy)

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

New issue 3387: add_entity and before_compile event
https://bitbucket.org/zzzeek/sqlalchemy/issue/3387/add_entity-and-before_compile-event

jaredks:

When adding an entity to a query "before_compile" the query is correctly executed but the row processor(s) used in `orm.loading.instances` still refer to the original query object - specifically the `_entities` list.

So if you had say related `User` and `Address` models and some query,


```
#!python

q = sess.query(User).filter_by(first_name='John')
```


and a listener,


```
#!python

@event.listens_for(SomeQuery, 'before_compile', retval=True)
def change_query(query):
    return query.add_entity(Address).join(Address)
```


You would expect the query to yield `KeyedTuple` but instead `User` entity is used.

A potential solution would be to change `Query._execute_and_instances` to return,


```
#!python

loading.instances(querycontext.query, result, query context)
```


instead of,


```
#!python

loading.instances(self, result, querycontext)
```

But this change may have other side-effects?