[Sqlalchemy-tickets] Issue #3432: Can't use postgresql_ops for expressions (zzzeek/sqlalchemy)

[Wichert A. <iss...@bi...>]

New issue 3432: Can't use postgresql_ops for expressions
https://bitbucket.org/zzzeek/sqlalchemy/issue/3432/cant-use-postgresql_ops-for-expressions

Wichert Akkerman:

I need to build a trigram index for a set of columns in PostgreSQL. The SQL statement looks like this:

```sql
CREATE INDEX trgm_idx ON my_table USING gist 
(language, (coalesce(col1, '') || ' ' || coalesce(col2, '')) gist_trgm_ops);
```

My attempt to do this using SQLAlchemy looks like this:

```python
schema.Index('trgm_idx', 
    Mytable.language,
    (func.coalesce(Mytable.col1, '') + ' ' + func.coalesce(MyTable.col2, '')).label('foo'),
    postgresql_ops={
        'foo': 'pg_trgm',
    },
    postgresql_using='gist')
```

I use `label()` just to get a key I can pass to `postgresql_ops`. Unfortunately this does not work. This is the generated SQL:

```
CREATE INDEX trgm_idx ON my_table USING gist (language, (coalesce(col1, '') || ' ' || coalesce(col2, '')))
```