[Sqlalchemy-tickets] Issue #3828: Postgres multi-value insert fails with enum-type columns and on_conflict statement (zzzeek/sqlalchemy)

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

New issue 3828: Postgres multi-value insert fails with enum-type columns and on_conflict statement
https://bitbucket.org/zzzeek/sqlalchemy/issues/3828/postgres-multi-value-insert-fails-with

Basti:

When creating a (postgres specific) insert statement for a table with a enum typed column, the compilation of the statement fails if the enum-column is used in the `on_conflict` where-condition.

Minimal Example:

```
#!python
import sqlalchemy as _sa
from sqlalchemy.dialects import postgresql as t
from st.db.schema import PrimaryKey as PrimaryKey

test_table = _sa.Table(u'test_table', _sa.MetaData(),
    _sa.Column('id', t.INTEGER, nullable=False, autoincrement=False),
    _sa.Column('state', t.ENUM(u'1st', u'2nd', u'3rd'), nullable=False),
)

PrimaryKey(
    test_table.c.id, 
    name=u'test_table_pkey',
)

PG_INSERT = t.insert

values = [dict(id=100, state=u'1st'),
          dict(id=200, state=u'1st')]

stmt = PG_INSERT(test_table, values)

stmt = stmt.on_conflict_do_update(
    index_elements=['id'], 
    set_=dict(id=1000), 
    where=(stmt.excluded.state == '2nd')
)

str(stmt.compile(dialect=t.dialect()))
# Fails with 'sqlalchemy.exc.CompileError: Bind parameter '%(4378430864 state)s'
# conflicts with unique bind parameter of the same name'
```

If instead of a list of values, a single dict is used in the `insert()` call everything is compiled correctly. Also multi-values work if the where-condition is replaced with any other (non-enum) column.