Improve RelatedField and callable default handling

- callable default values will now be properly called 
  - PrimaryKeyRelatedField and SlugRelatedField will now return an appropriate type based on the relation model's Field
  - mock views now have a request object bound even when public is True

testproj/snippets/migrations/0002_auto_20171205_0505.py

@@ -1,27 +0,0 @@
-# Generated by Django 2.0 on 2017-12-05 04:05
-
-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),
-        ('snippets', '0001_initial'),
-    ]
-
-    operations = [
-        migrations.AddField(
-            model_name='snippet',
-            name='owner',
-            field=models.ForeignKey(default='', on_delete=django.db.models.deletion.CASCADE, related_name='snippets', to=settings.AUTH_USER_MODEL),
-            preserve_default=False,
-        ),
-        migrations.AlterField(
-            model_name='snippet',
-            name='code',
-            field=models.TextField(help_text='code model help text'),
-        ),
-    ]

testproj/snippets/serializers.py

@@ -1,3 +1,4 @@
+from django.contrib.auth import get_user_model
 from rest_framework import serializers
 
 from snippets.models import Snippet, LANGUAGE_CHOICES, STYLE_CHOICES
@@ -23,7 +24,18 @@ class SnippetSerializer(serializers.Serializer):
     create: docstring for create from serializer classdoc
     """
     id = serializers.IntegerField(read_only=True, help_text="id serializer help text")
-    owner = serializers.ReadOnlyField(source='owner.username')
+    owner = serializers.PrimaryKeyRelatedField(
+        queryset=get_user_model().objects.all(),
+        default=serializers.CurrentUserDefault(),
+        help_text="The ID of the user that created this snippet; if none is provided, "
+                  "defaults to the currently logged in user."
+    )
+    owner_as_string = serializers.PrimaryKeyRelatedField(
+        help_text="The ID of the user that created this snippet.",
+        pk_field=serializers.CharField(help_text="this help text should not show up"),
+        read_only=True,
+        source='owner',
+    )
     title = serializers.CharField(required=False, allow_blank=True, max_length=100)
     code = serializers.CharField(style={'base_template': 'textarea.html'})
     linenos = serializers.BooleanField(required=False)
@@ -31,7 +43,8 @@ class SnippetSerializer(serializers.Serializer):
     styles = serializers.MultipleChoiceField(choices=STYLE_CHOICES, default=['friendly'])
     lines = serializers.ListField(child=serializers.IntegerField(), allow_empty=True, allow_null=True, required=False)
     example_projects = serializers.ListSerializer(child=ExampleProjectSerializer(), read_only=True)
-    difficulty_factor = serializers.FloatField(help_text="this is here just to test FloatField", read_only=True)
+    difficulty_factor = serializers.FloatField(help_text="this is here just to test FloatField",
+                                               read_only=True, default=lambda: 6.9)
 
     def create(self, validated_data):
         """
@@ -39,6 +52,7 @@ class SnippetSerializer(serializers.Serializer):
         """
         del validated_data['styles']
         del validated_data['lines']
+        del validated_data['difficulty_factor']
         return Snippet.objects.create(**validated_data)
 
     def update(self, instance, validated_data):