From e354dd10f8c3cc19e9cb3a6536f98798ba3538f1 Mon Sep 17 00:00:00 2001
From: Mike Bayer <mike_mp@zzzcomputing.com>
Date: Sun, 23 Sep 2012 12:42:38 -0400
Subject:   - [feature] An explicit error is raised when     a
 ForeignKeyConstraint() that was     constructed to refer to multiple remote
 tables     is first used. [ticket:2455]

---
 lib/sqlalchemy/schema.py | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

(limited to 'lib/sqlalchemy/schema.py')

diff --git a/lib/sqlalchemy/schema.py b/lib/sqlalchemy/schema.py
index 1c535461a..fdff37d3f 100644
--- a/lib/sqlalchemy/schema.py
+++ b/lib/sqlalchemy/schema.py
@@ -1391,10 +1391,10 @@ class ForeignKey(SchemaItem):
                 schema = parenttable.metadata.schema
 
             if (len(m) == 1):
-                tname   = m.pop()
+                tname = m.pop()
             else:
                 colname = m.pop()
-                tname   = m.pop()
+                tname = m.pop()
 
             if (len(m) > 0):
                 schema = '.'.join(m)
@@ -1403,11 +1403,24 @@ class ForeignKey(SchemaItem):
                 raise exc.NoReferencedTableError(
                     "Foreign key associated with column '%s' could not find "
                     "table '%s' with which to generate a "
-                    "foreign key to target column '%s'" % (self.parent, tname, colname),
+                    "foreign key to target column '%s'" %
+                    (self.parent, tname, colname),
                     tname)
             table = Table(tname, parenttable.metadata,
                           mustexist=True, schema=schema)
 
+            if not hasattr(self.constraint, '_referred_table'):
+                self.constraint._referred_table = table
+            elif self.constraint._referred_table is not table:
+                raise exc.ArgumentError(
+                    'ForeignKeyConstraint on %s(%s) refers to '
+                    'multiple remote tables: %s and %s' % (
+                    parenttable,
+                    self.constraint._col_description,
+                    self.constraint._referred_table,
+                    table
+                ))
+
             _column = None
             if colname is None:
                 # colname is None in the case that ForeignKey argument
@@ -2167,6 +2180,10 @@ class ForeignKeyConstraint(Constraint):
             self._set_parent_with_dispatch(columns[0].table)
 
 
+    @property
+    def _col_description(self):
+        return ", ".join(self._elements)
+
     @property
     def columns(self):
         return self._elements.keys()
@@ -2177,6 +2194,7 @@ class ForeignKeyConstraint(Constraint):
 
     def _set_parent(self, table):
         super(ForeignKeyConstraint, self)._set_parent(table)
+
         for col, fk in self._elements.iteritems():
             # string-specified column names now get
             # resolved to Column objects
-- 
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