Don't automatically collapse '' and None for keys.

Many databases treat these differently, including MySQL, and there are
legitimate cases where `''` is a valid value in key fields. This change
updates both the ForeignKey class and the row cache to check the
connection feature `interprets_empty_strings_as_nulls` and will collapse
these two values if that's `True`, otherwise it will treat them
differently.

django/db/models/fields/related.py

@@ -1034,7 +1034,9 @@ def get_default(self):
         return field_default
 
     def get_db_prep_save(self, value, connection):
-        if value == '' or value == None:
+        if (value == None or
+            (connection.features.interprets_empty_strings_as_nulls 
+             and value == '')):
             return None
         else:
             return self.rel.get_related_field().get_db_prep_save(value,

django/db/models/query.py

@@ -1441,9 +1441,12 @@ def get_cached_row(row, index_start, using,  klass_info, offset=0,
 
 
     fields = row[index_start : index_start + field_count]
-    # If the pk column is None (or the Oracle equivalent ''), then the related
+    # If the pk column is None (or the equivalent '' in the case the
+    # connection interprets empty strings as nulls), then the related
     # object must be non-existent - set the relation to None.
-    if fields[pk_idx] == None or fields[pk_idx] == '':
+    if (fields[pk_idx] == None or
+        (connections[using].features.interprets_empty_strings_as_nulls 
+         and fields[pk_idx] == '')):
         obj = None
     elif field_names:
         fields = list(fields)

tests/modeltests/model_forms/models.py

@@ -272,3 +272,10 @@ class FlexibleDatePost(models.Model):
     slug = models.CharField(max_length=50, unique_for_year='posted', blank=True)
     subtitle = models.CharField(max_length=50, unique_for_month='posted', blank=True)
     posted = models.DateField(blank=True, null=True)
+
+class EmptyKeySource(models.Model):
+    name = models.CharField(max_length=50, unique=True, null=False, blank=True, db_index=True)
+
+class EmptyKeySink(models.Model):
+    title = models.CharField(max_length=50, unique=True, null=False, blank=True, db_index=True)
+    source = models.ForeignKey(EmptyKeySource, to_field='name', blank=True, null=False)

tests/modeltests/model_forms/tests.py

@@ -18,7 +18,8 @@
     Category, CommaSeparatedInteger, CustomFieldForExclusionModel, DerivedBook,
     DerivedPost, ExplicitPK, FlexibleDatePost, ImprovedArticle,
     ImprovedArticleWithParentLink, Inventory, PhoneNumber, Post, Price,
-    Product, TextFile, Writer, WriterProfile, test_images)
+    Product, TextFile, Writer, WriterProfile, test_images,
+    EmptyKeySource, EmptyKeySink)
 
 if test_images:
     from .models import ImageFile, OptionalImageFile
@@ -1524,3 +1525,12 @@ def test_model_field_that_returns_none_to_exclude_itself_with_explicit_fields(se
                          ['name'])
         self.assertHTMLEqual(six.text_type(CustomFieldForExclusionForm()),
                          '''<tr><th><label for="id_name">Name:</label></th><td><input id="id_name" type="text" name="name" maxlength="10" /></td></tr>''')
+
+    def test_foreignkeys_which_use_to_field_and_blank_string_key(self):
+        source_n = EmptyKeySource.objects.create(name="")
+        source_a = EmptyKeySource.objects.create(name="a")
+        source_b = EmptyKeySource.objects.create(name="b")
+
+        sink_n = EmptyKeySink.objects.create(title="Hello", source=source_n)
+
+        self.assertEqual(sink_n.source, source_n)