[Sqlalchemy-tickets] Issue #3855: Comparator on Variant field (zzzeek/sqlalchemy)

[Jiří K. <iss...@bi...>]

New issue 3855: Comparator on Variant field
https://bitbucket.org/zzzeek/sqlalchemy/issues/3855/comparator-on-variant-field

Jiří Kunčar:

The comparator factory for Variant field doesn't lookup the engine specific implementation.

```python
    @property
    def comparator_factory(self):
        """express comparison behavior in terms of the base type"""
        return self.impl.comparator_factory
```

I would expect something like:

```python
    @property
    def comparator_factory(self):
        """express comparison behavior in terms of the base type"""
        return self.load_dialect_impl(FIXME).comparator_factory
```

There is a workaround which more or less works for me but it is not ideal:

```python
    TestJSONType = sa.JSON().with_variant(
        postgresql.JSONB(), 'postgresql'
    ).with_variant(
        mysql.JSON(), 'mysql'
    )

    class TestJSON(db.Model):
        __tablename__ = 'test_json'

        pk = sa.Column(sa.Integer, primary_key=True)
        js = sa.Column(TestJSONType)
```

```python
    from werkzeug.local import LocalProxy

    def variant_comparator(column):
        """Dialect specific comparator."""
        return column.type.load_dialect_impl(
            db.engine.dialect
        ).comparator_factory(column)

    TestJSON.js.comparator = LocalProxy(lambda: variant_comparator(TestJSON.js))
```