Raise if ForeignKeyConstraint created with different numbers of

local and remote columns.

An :class:`.ArgumentError` is now raised if a
:class:`.ForeignKeyConstraint` object is created with a mismatched
number of "local" and "remote" columns, which otherwise causes the
internal state of the constraint to be incorrect.   Note that this
also impacts the condition where a dialect's reflection process
produces a mismatched set of columns for a foreign key constraint.

Downstream DB2 dialect has been reported as potentially causing this
scenario.

Change-Id: Id51c34a6c43749bb582639f9c1dc28723482f0e5
Fixes: #3949
References: #3998

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index a9aee5883..4eb095196 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -2883,6 +2883,22 @@ class ForeignKeyConstraint(ColumnCollectionConstraint):
         self.use_alter = use_alter
         self.match = match
 
+        if len(set(columns)) != len(refcolumns):
+            if len(set(columns)) != len(columns):
+                # e.g. FOREIGN KEY (a, a) REFERENCES r (b, c)
+                raise exc.ArgumentError(
+                    "ForeignKeyConstraint with duplicate source column "
+                    "references are not supported."
+                )
+            else:
+                # e.g. FOREIGN KEY (a) REFERENCES r (b, c)
+                # paraphrasing https://www.postgresql.org/docs/9.2/static/\
+                # ddl-constraints.html
+                raise exc.ArgumentError(
+                    "ForeignKeyConstraint number "
+                    "of constrained columns must match the number of "
+                    "referenced columns.")
+
         # standalone ForeignKeyConstraint - create
         # associated ForeignKey objects which will be applied to hosted
         # Column objects (in col.foreign_keys), either now or when attached