Ticket #11702: related.patch

django/db/models/fields/related.py

@@ -12 +12 @@
 from django.utils.functional import curry
 from django.core import exceptions
 from django import forms
+from django.core.exceptions import FieldError
 
 
 RECURSIVE_RELATIONSHIP_CONSTANT = 'self'
@@ -752 +753 @@
 class ForeignKey(RelatedField, Field):
     empty_strings_allowed = False
     default_error_messages = {
-        'invalid': _('Model %(model)s with pk %(pk)r does not exist.')
+        'invalid': _('Model %(model)s with pk %(pk)r does not exist.'),
+        'improperly_configured': _('Field \'%(field)s\' under model \'%(model)s\' must have a unique=True constraint.'),
     }
     description = _("Foreign Key (type determined by related field)")
     def __init__(self, to, to_field=None, rel_class=ManyToOneRel, **kwargs):
@@ -766 +768 @@
             # the to_field during FK construction. It won't be guaranteed to
             # be correct until contribute_to_class is called. Refs #12190.
             to_field = to_field or (to._meta.pk and to._meta.pk.name)
+            # Make sure we have a unique=True constraint in the ForeignKey field if to_field is set.
+            if to_field:
+                try:
+                    assert to._meta.get_field(to_field).unique, \
+                           "Field %s under model %s must have a unique=True constraint." % (to_field, to.__class__.__name__)
+                except:
+                    raise FieldError(self.default_error_messages['improperly_configured'] % {
+                        'field': to_field, 'model': to._meta.object_name})
+
         kwargs['verbose_name'] = kwargs.get('verbose_name', None)
 
         kwargs['rel'] = rel_class(to, to_field,