No mention of index_together on the GenericForeignKey section

[Aidan Lister]

We've been troubleshooting a performance issue and noticed that there was no index on an important GenericForiegnKey.

Once we'd resolved that, we noticed a contrib app had no index either (django reversion).

It seems like this should be mentioned in the documentation here:

​https://docs.djangoproject.com/en/1.9/ref/contrib/contenttypes/#generic-relations

Suggested wording:

If you plan to search for child objects which have a generic relation to your parent object, e.g. find all tags linked to a piece of content, then you should consider adding a database index on the content type and object ID using index_together (​https://docs.djangoproject.com/en/1.9/ref/models/options/#index-together). Django does not do this for you automatically as it does with ForeignKey's.

In the example above, this would look like:

    class Meta:
        index_together = [
            ["content_type", "object_id"],
        ]