- repair a regression caused by #3282, where we no longer were

applying any topological sort to tables on SQLite.  See the
changelog for details, but we now continue to sort
tables for SQLite on DROP, prohibit the sort from considering
alter, and only warn if we encounter an unresolvable cycle, in
which case, then we forego the ordering.  use_alter as always
is used to break such a cycle.
fixes #3378

1 files changed, 32 insertions, 14 deletions

diff --git a/lib/sqlalchemy/sql/ddl.py b/lib/sqlalchemy/sql/ddl.py
index bbac6456e..a0841b13c 100644
--- a/lib/sqlalchemy/sql/ddl.py
+++ b/lib/sqlalchemy/sql/ddl.py
@@ -803,32 +803,50 @@ class SchemaDropper(DDLBase):
             tables = list(metadata.tables.values())
 
         try:
+            unsorted_tables = [t for t in tables if self._can_drop_table(t)]
             collection = reversed(
                 sort_tables_and_constraints(
-                    [t for t in tables if self._can_drop_table(t)],
-                    filter_fn=
-                    lambda constraint: True if not self.dialect.supports_alter
-                    else False if constraint.name is None
+                    unsorted_tables,
+                    filter_fn=lambda constraint: False
+                    if not self.dialect.supports_alter
+                    or constraint.name is None
                     else None
                 )
             )
         except exc.CircularDependencyError as err2:
-            util.raise_from_cause(
-                exc.CircularDependencyError(
-                    err2.args[0],
-                    err2.cycles, err2.edges,
-                    msg="Can't sort tables for DROP; an "
+            if not self.dialect.supports_alter:
+                util.warn(
+                    "Can't sort tables for DROP; an "
                     "unresolvable foreign key "
-                    "dependency exists between tables: %s.  Please ensure "
-                    "that the ForeignKey and ForeignKeyConstraint objects "
-                    "involved in the cycle have "
-                    "names so that they can be dropped using DROP CONSTRAINT."
+                    "dependency exists between tables: %s, and backend does "
+                    "not support ALTER.  To restore at least a partial sort, "
+                    "apply use_alter=True to ForeignKey and "
+                    "ForeignKeyConstraint "
+                    "objects involved in the cycle to mark these as known "
+                    "cycles that will be ignored."
                     % (
                         ", ".join(sorted([t.fullname for t in err2.cycles]))
                     )
+                )
+                collection = [(t, ()) for t in unsorted_tables]
+            else:
+                util.raise_from_cause(
+                    exc.CircularDependencyError(
+                        err2.args[0],
+                        err2.cycles, err2.edges,
+                        msg="Can't sort tables for DROP; an "
+                        "unresolvable foreign key "
+                        "dependency exists between tables: %s.  Please ensure "
+                        "that the ForeignKey and ForeignKeyConstraint objects "
+                        "involved in the cycle have "
+                        "names so that they can be dropped using "
+                        "DROP CONSTRAINT."
+                        % (
+                            ", ".join(sorted([t.fullname for t in err2.cycles]))
+                        )
 
+                    )
                 )
-            )
 
         seq_coll = [
             s