Change visit name for ColumnElement

No SQLA built-in subclasses ColumnElement without specifying
an alternate visit_name, and user defined ColumnElement
subclasses should avoid being treated like ColumnClause,
e.g. where a Table is present.

Fixes: #4142
Change-Id: I15ed09ba8bdebae4cb0c7e5e5df3f59351477577

3 files changed, 24 insertions, 3 deletions

diff --git a/doc/build/changelog/unreleased_12/4142.rst b/doc/build/changelog/unreleased_12/4142.rst
new file mode 100644
index 000000000..be5ac2fb7
--- /dev/null
+++ b/doc/build/changelog/unreleased_12/4142.rst
@@ -0,0 +1,9 @@
+.. change::
+    :tags: bug, sql
+    :tickets: 4142
+
+    Changed the "visit name" of :class:`.ColumnElement` from "column" to
+    "column_element", so that when this element is used as the basis for a
+    user-defined SQL element, it is not assumed to behave like a table-bound
+    :class:`.ColumnClause` when processed by various SQL traversal utilities,
+    as are commonly used by the ORM.
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index 3b2bcb4ff..2cc1d9c42 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -571,7 +571,7 @@ class ColumnElement(operators.ColumnOperators, ClauseElement):
 
     """
 
-    __visit_name__ = 'column'
+    __visit_name__ = 'column_element'
     primary_key = False
     foreign_keys = []
 
diff --git a/test/sql/test_utils.py b/test/sql/test_utils.py
index 84e7ad732..bd8368cd2 100644
--- a/test/sql/test_utils.py
+++ b/test/sql/test_utils.py
@@ -1,8 +1,9 @@
-from sqlalchemy.testing import fixtures, is_true, is_false
+from sqlalchemy.testing import fixtures, is_true, is_false, eq_
 from sqlalchemy import MetaData, Table, Column, Integer, String
 from sqlalchemy import and_, or_, bindparam
-from sqlalchemy.sql.elements import ClauseList
+from sqlalchemy.sql.elements import ClauseList, ColumnElement
 from sqlalchemy.sql import operators
+from sqlalchemy.sql import util as sql_util
 
 
 class CompareClausesTest(fixtures.TestBase):
@@ -106,3 +107,14 @@ class CompareClausesTest(fixtures.TestBase):
         is_false(b1.compare(b8))
         is_false(b8.compare(b9))
         is_true(b8.compare(b8))
+
+
+class MiscTest(fixtures.TestBase):
+    def test_column_element_no_visit(self):
+        class MyElement(ColumnElement):
+            pass
+
+        eq_(
+            sql_util.find_tables(MyElement(), check_columns=True),
+            []
+        )