[1.8.x] Fixed #24343 -- Ensure db converters are used for foreign keys.

Joint effort between myself, Josh, Anssi and Shai.

Conflicts:
	django/db/models/query.py
	tests/model_fields/models.py

Backport of 4755f8fc25331c739a6f932cc8aba0cc9e62e352 from master.

5 files changed, 31 insertions, 14 deletions

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 8e5c4aa533..2e41803a1a 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -585,10 +585,10 @@ class Random(ExpressionNode):
 
 
 class Col(ExpressionNode):
-    def __init__(self, alias, target, source=None):
-        if source is None:
-            source = target
-        super(Col, self).__init__(output_field=source)
+    def __init__(self, alias, target, output_field=None):
+        if output_field is None:
+            output_field = target
+        super(Col, self).__init__(output_field=output_field)
         self.alias, self.target = alias, target
 
     def __repr__(self):
@@ -606,7 +606,10 @@ class Col(ExpressionNode):
         return [self]
 
     def get_db_converters(self, connection):
-        return self.output_field.get_db_converters(connection)
+        if self.target == self.output_field:
+            return self.output_field.get_db_converters(connection)
+        return (self.output_field.get_db_converters(connection) +
+                self.target.get_db_converters(connection))
 
 
 class Ref(ExpressionNode):
diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 31b0f26141..19f090863b 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -333,12 +333,12 @@ class Field(RegisterLookupMixin):
             ]
         return []
 
-    def get_col(self, alias, source=None):
-        if source is None:
-            source = self
-        if alias != self.model._meta.db_table or source != self:
+    def get_col(self, alias, output_field=None):
+        if output_field is None:
+            output_field = self
+        if alias != self.model._meta.db_table or output_field != self:
             from django.db.models.expressions import Col
-            return Col(alias, self, source)
+            return Col(alias, self, output_field)
         else:
             return self.cached_col
 
diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index a8e4a3156d..3410e5c994 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -1991,6 +1991,20 @@ class ForeignKey(ForeignObject):
     def db_parameters(self, connection):
         return {"type": self.db_type(connection), "check": []}
 
+    def convert_empty_strings(self, value, connection, context):
+        if (not value) and isinstance(value, six.string_types):
+            return None
+        return value
+
+    def get_db_converters(self, connection):
+        converters = super(ForeignKey, self).get_db_converters(connection)
+        if connection.features.interprets_empty_strings_as_nulls:
+            converters += [self.convert_empty_strings]
+        return converters
+
+    def get_col(self, alias, output_field=None):
+        return super(ForeignKey, self).get_col(alias, output_field or self.related_field)
+
 
 class OneToOneField(ForeignKey):
     """
diff --git a/django/db/models/query.py b/django/db/models/query.py
index 0ec3cfd730..b055205772 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -243,7 +243,7 @@ class QuerySet(object):
         model_cls = klass_info['model']
         select_fields = klass_info['select_fields']
         model_fields_start, model_fields_end = select_fields[0], select_fields[-1] + 1
-        init_list = [f[0].output_field.attname
+        init_list = [f[0].target.attname
                      for f in select[model_fields_start:model_fields_end]]
         if len(init_list) != len(model_cls._meta.concrete_fields):
             init_set = set(init_list)
@@ -1699,7 +1699,7 @@ class RelatedPopulator(object):
             self.cols_start = select_fields[0]
             self.cols_end = select_fields[-1] + 1
             self.init_list = [
-                f[0].output_field.attname for f in select[self.cols_start:self.cols_end]
+                f[0].target.attname for f in select[self.cols_start:self.cols_end]
             ]
             self.reorder_for_init = None
         else:
@@ -1708,7 +1708,7 @@ class RelatedPopulator(object):
             ]
             reorder_map = []
             for idx in select_fields:
-                field = select[idx][0].output_field
+                field = select[idx][0].target
                 init_pos = model_init_attnames.index(field.attname)
                 reorder_map.append((init_pos, field.attname, idx))
             reorder_map.sort()
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index f99b58b84f..344b298688 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -1542,7 +1542,7 @@ class Query(object):
         # database from tripping over IN (...,NULL,...) selects and returning
         # nothing
         col = query.select[0]
-        select_field = col.field
+        select_field = col.target
         alias = col.alias
         if self.is_nullable(select_field):
             lookup_class = select_field.get_lookup('isnull')