[Sqlalchemy-tickets] Issue #3620: New query doesn't expire column_property with bindparam (zzzeek/sqlalchemy)

[Lukas S. <iss...@bi...>]

New issue 3620: New query doesn't expire column_property with bindparam
https://bitbucket.org/zzzeek/sqlalchemy/issues/3620/new-query-doesnt-expire-column_property

Lukas Siemon:

The column_property seems to be cached even when a new query is issued. Is doesn't matter if I use deferred or not. The expunge is obviously really ugly as a workaround. Is this something that could potentially be fixed or is there a better way of "flushing" the cache?

We are using this pattern in many places in our application. However I only noticed today that the value gets cached. I would very much prefer a generic solution (i.e. where the model is declared and not where the query is generated).

Test case:
```
#!python

import unittest
from sqlalchemy import (Column, Integer, String, select, bindparam)
from sqlalchemy.orm import (column_property, undefer)
from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy


class TestDynamicBindparam(unittest.TestCase):

    def setUp(self):
        # -- create all the database models
        app = Flask(__name__)
        app.config['SQLALCHEMY_DATABASE_URI'] = ("sqlite://")
        db = SQLAlchemy(app)

        class Label(db.Model):
            __tablename__ = 'label'
            id = Column(Integer, primary_key=True, nullable=False)

            write_only = column_property(
                select(['-w-' == bindparam('perm', value=None, type_=String)]),
                deferred=True
            )

        db.drop_all()
        db.create_all()

        self.Label = Label
        self.db = db

    def test_dynamic_bindparam(self):
        label = self.Label()
        self.db.session.add(label)
        self.db.session.commit()
        self.assertEqual(label.id, 1)
        self.assertIsNone(label.write_only)

        label = label.query.options(
            undefer(self.Label.write_only)
        ).params(perm='-w-').one()
        self.assertIsNone(label.write_only)  # this should be True (!)

        self.db.session.expunge(label)

        label = label.query.options(
            undefer(self.Label.write_only)
        ).params(perm='-w-').one()
        self.assertTrue(label.write_only)

        label = label.query.options(
            undefer(self.Label.write_only)
        ).params(perm='---').one()
        self.assertTrue(label.write_only)  # this should be False (!)

```

Responsible: zzzeek