[Sqlalchemy-tickets] Issue #3645: UPDATE on a Join creates duplicate tables (zzzeek/sqlalchemy)

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

New issue 3645: UPDATE on a Join creates duplicate tables
https://bitbucket.org/zzzeek/sqlalchemy/issues/3645/update-on-a-join-creates-duplicate-tables

immerrr:

It is probably unexpected that it actually works, but MySQL can do that (and StackOverflow suggests that MS SQL can, too). Here's a script to reproduce:

```
import sqlalchemy as sa

metadata = sa.MetaData()


t1 = sa.Table(
    't1', metadata,
    sa.Column('key1', sa.Integer),
    sa.Column('ref1', sa.Integer),
    sa.Column('val1', sa.Integer),
    sa.ForeignKeyConstraint(['ref1'], ['t2.key2']),
)


t2 = sa.Table(
    't2', metadata,
    sa.Column('key2', sa.Integer),
    sa.Column('val2', sa.Integer),
)

engine = sa.create_engine('mysql+mysqlconnector://')


def to_str(stmt):
    return str(stmt.compile(bind=engine,
                            compile_kwargs={'literal_binds': True}))
```

In the end I see
```
>>> to_str(
...     sa.update(t1.join(t2),
...               values={t1.c.val1: 'foo'},
...               whereclause=(t2.c.val2 == 'foobar')))
...
'UPDATE t1 INNER JOIN t2 ON t2.key2 = t1.ref1, t2 SET t1.val1=%(val1)s WHERE t2.val2 = %(val2_1)s'
```

Note, that there's a second `t2` table in the *table_references* part of the query.

This patch seems to have fixed my case, but I can't seem to run tests out of the box, so I don't know if it broke anything else:
```
index 7b506f9..fe9f20b 100644
--- a/lib/sqlalchemy/sql/dml.py
+++ b/lib/sqlalchemy/sql/dml.py
@@ -766,7 +766,7 @@ class Update(ValuesBase):
         # TODO: this could be made memoized
         # if the memoization is reset on each generative call.
         froms = []
-        seen = set([self.table])
+        seen = set(_from_objects(self.table))
 
         if self._whereclause is not None:
             for item in _from_objects(self._whereclause):
```