[Sqlalchemy-tickets] Issue #4194: can't specify select-bound versioning column (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 4194: can't specify select-bound versioning column
https://bitbucket.org/zzzeek/sqlalchemy/issues/4194/cant-specify-select-bound-versioning

Michael Bayer:

this is dependent on #4193 being resolved

```
#!python


from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()


version_table = Table(
    'version_table', Base.metadata,
    Column('id', Integer, primary_key=True),
    Column('version_id', Integer, nullable=False),
    Column('value', String(40), nullable=False))

current = version_table.select().\
    where(version_table.c.id > 0).alias('current_table')


class Versioned(Base):
    __table__ = current

    __mapper_args__ = {
        # won't insert,
        # "version_id_col": current.c.version_id

        # inserts assuming we fixed #4193, but then expires the column
        "version_id_col": version_table.c.version_id
    }

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)
v1 = Versioned(value='x')
s.add(v1)
s.flush()

print("-------------------")
# emits SQL if we mapped to table-bound column
print(v1.version_id)
```

this is because:


```
#!diff

diff --git a/lib/sqlalchemy/orm/persistence.py b/lib/sqlalchemy/orm/persistence.py
index dc0ae1c38..07fb50d6e 100644
--- a/lib/sqlalchemy/orm/persistence.py
+++ b/lib/sqlalchemy/orm/persistence.py
@@ -429,6 +429,10 @@ def _collect_insert_commands(
         else:
             has_all_defaults = has_all_pks = True
 
+        # HERE vvvvvvvvvvvvv
+        # version_id_col needs to be the table bound column.
+        # the only way it could "work" the other way would be if we maintain
+        # corresponding column and all that.
         if mapper.version_id_generator is not False \
                 and mapper.version_id_col is not None and \
                 mapper.version_id_col in mapper._cols_by_table[table]:
@@ -1081,7 +1085,6 @@ def _finalize_insert_update_commands(base_mapper, uowtransaction, states):
 
     """
     for state, state_dict, mapper, connection, has_identity in states:
-
         if mapper._readonly_props:
             readonly = state.unmodified_intersection(
                 [

```