fix: ensure config has no app when constructor finishes

had to move `make_engine_from_config()` out of app handler and define
as a separate function, so that `get_engines()` did not need to
instantiate the app handler.  because if it did, then config
extensions would lose the ability to set a default app handler - er,
they could do it but it would be ignored

tests/db/test_conf.py

@@ -7,6 +7,8 @@ from unittest import TestCase
 
 import sqlalchemy as sa
 from sqlalchemy import orm
+from sqlalchemy.engine import Engine
+from sqlalchemy.pool import NullPool
 
 from wuttjamaican.db import conf
 from wuttjamaican.conf import WuttaConfig
@@ -93,3 +95,38 @@ class TestGetSetting(TestCase):
     def test_missing_value(self):
         value = conf.get_setting(self.session, 'foo')
         self.assertIsNone(value)
+
+
+class TestMakeEngineFromConfig(TestCase):
+
+    def test_basic(self):
+        engine = conf.make_engine_from_config({
+            'sqlalchemy.url': 'sqlite://',
+        })
+        self.assertIsInstance(engine, Engine)
+
+    def test_poolclass(self):
+
+        engine = conf.make_engine_from_config({
+            'sqlalchemy.url': 'sqlite://',
+        })
+        self.assertNotIsInstance(engine.pool, NullPool)
+
+        engine = conf.make_engine_from_config({
+            'sqlalchemy.url': 'sqlite://',
+            'sqlalchemy.poolclass': 'sqlalchemy.pool:NullPool',
+        })
+        self.assertIsInstance(engine.pool, NullPool)
+
+    def test_pool_pre_ping(self):
+
+        engine = conf.make_engine_from_config({
+            'sqlalchemy.url': 'sqlite://',
+        })
+        self.assertFalse(engine.pool._pre_ping)
+
+        engine = conf.make_engine_from_config({
+            'sqlalchemy.url': 'sqlite://',
+            'sqlalchemy.pool_pre_ping': 'true',
+        })
+        self.assertTrue(engine.pool._pre_ping)