[Sqlalchemy-tickets] Issue #3193: CompoundSelect not supporting CTEs with inserts (zzzeek/sqlalchemy)

[Konsta V. <iss...@bi...>]

New issue 3193: CompoundSelect not supporting CTEs with inserts
https://bitbucket.org/zzzeek/sqlalchemy/issue/3193/compoundselect-not-supporting-ctes-with

Konsta Vesterinen:

I'm trying to do atomic SELECT or INSERT as explained here: http://stackoverflow.com/questions/6722344/select-or-insert-a-row-in-one-command/6722460#6722460

I'v been able to construct the query, however when I try to execute this query it gives me: 

AttributeError: 'CompoundSelect' object has no attribute '_returning'

Here is a minimal test case for reproducing the issue:
```
#!python

import sqlalchemy as sa
from sqlalchemy import create_engine
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.sql.expression import CTE, exists

Model = declarative_base()


dns = 'postgres://postgres@localhost/my_test_db'

engine = create_engine(dns)
# engine.echo = True
connection = engine.connect()


class MyModel(Model):
    __tablename__ = 'my_model'

    id = sa.Column(sa.Integer, autoincrement=True, primary_key=True)
    key = sa.Column(sa.Unicode(255), nullable=False)


sa.orm.configure_mappers()
Model.metadata.create_all(connection)

table = MyModel.__table__
value = 'some_key'

select = (
    sa.select(['*'], from_obj=table)
    .where(table.c.key == value)
)
insert = CTE(
    table.insert()
    .from_select(
        [table.c.key],
        sa.select([sa.text(value)])
        .where(~ exists(select))
    )
    .returning(sa.text('*')),
    name='new_row'
)
query = sa.select(['*'], from_obj=insert).union(select)
print connection.execute(query).fetchall()
```