follow relationships on ForeignKeyWidget

import_export/widgets.py

@@ -7,6 +7,7 @@ from django.utils import datetime_safe, timezone, six
 from django.utils.encoding import smart_text
 from django.utils.dateparse import parse_duration
 from django.conf import settings
+from django.core.exceptions import ObjectDoesNotExist
 
 try:
     from django.utils.encoding import force_text
@@ -343,7 +344,19 @@ class ForeignKeyWidget(Widget):
     def render(self, value, obj=None):
         if value is None:
             return ""
-        return getattr(value, self.field)
+
+        attrs = self.field.split('__')
+        for attr in attrs:
+            try:
+                value = getattr(value, attr, None)
+            except (ValueError, ObjectDoesNotExist):
+                # needs to have a primary key value before a many-to-many
+                # relationship can be used.
+                return None
+            if value is None:
+                return None
+
+        return value
 
 
 class ManyToManyWidget(Widget):

tests/core/migrations/0007_auto_20180628_0411.py

@@ -0,0 +1,34 @@
+# Generated by Django 2.0.2 on 2018-06-28 04:11
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('core', '0006_auto_20171130_0147'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Person',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+            ],
+        ),
+        migrations.CreateModel(
+            name='Role',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('user', models.OneToOneField(null=True, on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL)),
+            ],
+        ),
+        migrations.AddField(
+            model_name='person',
+            name='role',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='core.Role'),
+        ),
+    ]

tests/core/models.py

@@ -68,6 +68,14 @@ class Entry(models.Model):
     user = models.ForeignKey('auth.User', on_delete=models.CASCADE)
 
 
+class Role(models.Model):
+    user = models.OneToOneField('auth.User', on_delete=models.CASCADE, null=True)
+
+
+class Person(models.Model):
+    role = models.ForeignKey(Role, on_delete=models.CASCADE)
+
+
 class WithDefault(models.Model):
     name = models.CharField('Default', max_length=75, blank=True,
                             default='foo_bar')

tests/core/tests/test_resources.py

@@ -21,7 +21,7 @@ from import_export.resources import Diff
 
 from ..models import (
     Author, Book, Category, Entry, Profile, WithDefault, WithDynamicDefault,
-    WithFloatField,
+    WithFloatField, Person, Role
 )
 
 try:
@@ -935,6 +935,38 @@ if 'postgresql' in settings.DATABASES['default']['ENGINE']:
             self.assertListEqual(book_with_chapters.chapters, chapters)
 
 
+class ForeignKeyWidgetFollowRelationship(TestCase):
+    def setUp(self):
+        self.user = User.objects.create(username='foo')
+        self.role = Role.objects.create(user=self.user)
+        self.person = Person.objects.create(role=self.role)
+
+    def test_export(self):
+        class MyPersonResource(resources.ModelResource):
+            role = fields.Field(
+                column_name='role',
+                attribute='role',
+                widget=widgets.ForeignKeyWidget(Role, field='user__username')
+            )
+
+            class Meta:
+                model = Person
+                fields = ['id', 'role']
+
+        resource = MyPersonResource()
+        dataset = resource.export(Person.objects.all())
+        self.assertEqual(len(dataset), 1)
+        self.assertEqual(dataset[0][0], 'foo')
+
+        self.role.user = None
+        self.role.save()
+
+        resource = MyPersonResource()
+        dataset = resource.export(Person.objects.all())
+        self.assertEqual(len(dataset), 1)
+        self.assertEqual(dataset[0][0], None)
+
+
 class ManyRelatedManagerDiffTest(TestCase):
     fixtures = ["category", "book"]