Complex compound order_by django query

Question

I have the following two models for the title of Movies and TV:

Title
    - id
    - title
    - show_name (FK, Nullable)

TVShow
    - id
    - title

Here are some example data entries:

Episode_Title
- (1, "Terminator", NULL)
- (2, "ZZZ", 1) --> FK to TVShow (1, "Seinfeld")
- (3, "Abyss", NULL)

Here is how it should be sorted:

- Abyss (sort by episode title if show NULL)
- Seinfeld - ZZZ (sort by show title if show NOT NULL)
- Terminator (sort by title if show NULL)

How would I do this with a django QuerySet? What I have now, which is incorrect --

Title.objects.filter('title', 'show__title')

Answer 1

I think you want:

Title.objects.select_related().extra(
    select={'sort_title':"COALESCE(`tv_show`.`title`, `title`.`title`)"},
    order_by=['sort_title']
)

So you are adding a "virtual" field to the SELECT clause, COALESCE will give you the first non-null value, so sort_title will be the TV show's title if there is one, or the regular title. Then you can sort by the name you give it. The select_related() is so that the join is done in 1 query, I am not sure what the table names are but you can take it from there...