Fixed #13813 -- Comparison of DatabaseWrappers doesn't raise errors.

Patch from Lukasz Balcerzak.

django/db/backends/__init__.py

@@ -70,7 +70,9 @@ class BaseDatabaseWrapper(object):
         self._thread_ident = thread.get_ident()
 
     def __eq__(self, other):
-        return self.alias == other.alias
+        if isinstance(other, BaseDatabaseWrapper):
+            return self.alias == other.alias
+        return NotImplemented
 
     def __ne__(self, other):
         return not self == other

tests/db_backends/tests.py

+from django.test import TestCase
+from django.db.backends import BaseDatabaseWrapper
+
+
+class DummyDatabaseWrapper(BaseDatabaseWrapper):
+    pass
+
+
+class DummyObject(object):
+    alias = None
+
+
+class DbBackendTests(TestCase):
+    def test_compare_db_wrapper_with_another_object(self):
+        wrapper = BaseDatabaseWrapper({})
+        self.assertFalse(wrapper == 'not-a-db-wrapper')
+
+    def test_compare_db_wrapper_with_another_object_with_alias(self):
+        wrapper = BaseDatabaseWrapper({})
+        obj = DummyObject()
+        obj.alias = wrapper.alias = 'foobar'
+        self.assertFalse(wrapper == obj)
+
+    def test_negate_compare_db_wrapper_with_another_object(self):
+        wrapper = BaseDatabaseWrapper({})
+        self.assertTrue(wrapper != 'not-a-db-wrapper')
+
+    def test_compare_db_wrappers(self):
+        wrapper1 = DummyDatabaseWrapper({})
+        wrapper2 = BaseDatabaseWrapper({})
+
+        wrapper1.alias = wrapper2.alias = 'foo'
+        self.assertTrue(wrapper1 == wrapper2)
+
+        wrapper1.alias = 'bar'
+        self.assertFalse(wrapper1 == wrapper2)