Regex search option

Using https://docs.djangoproject.com/en/1.8/ref/models/querysets/#regex for search filter by starting field with '$'

Limit rendering of relational selects to max 1000 items by default.

Raise error when `source=` use on a child.

Closes #3292

Add import to example for filtering

`from rest_framework import filters` was missing from one of the examples

Fix typos in comments

Cached the field's root and context property

Resolved issue with rendering of nested serializers form when value is None

Update 2-requests-and-responses.md

just a small error, the trailing slash in the url should be omitted when adding a .suffix

Raise error if passed a serializer instance

Add a method to return the string value used in `choices` of a `RelatedField`

The `display_value` method returns a text type.