wagtail search_fields on snippet with foreign key - django

Question

I have a snippet which is a proxy of one of my standard django models.
search_fields works fine when filtering on standard fields, the problem is I can't seem to get foreign keys to work.
This page has an example on the bottom that shows how to create searchable snippets:
https://docs.wagtail.org/en/stable/topics/snippets.html
The main model has a field called "day" which is a foreign key to a Day-table. A day has a calendar_year, which I would like to be able to filter on while searching in the wagtail snippets area. in the def str method I'm able to display the name in the list, the search is the problem here.
Suggestions?
#register_snippet
class EventSnippet(index.Indexed, Event):
# We make a proxy model just to be able to add to this file or potentially if we want custom methods on it.
panels = [
FieldPanel('name'),
]
search_fields = [
index.SearchField('day__calendar_year', partial_match=True), # This prompts an error
index.SearchField('name', partial_match=True),
]
class Meta:
proxy = True
def __str__(self):
return f"{self.name} {self.day.calendar_year}"
When running python manage.py update_index i get the following warning:
EventSnippet.search_fields contains non-existent field 'day__calendar_year

Answer 1

You can't use complex lookups with double-underscores inside SearchField - search queries work by populating a central table (the search index) in advance with the data you're going to be searching on, which means you can't do arbitrary lookups and transformations on it like you would with a standard database query.
However, you can use any method or attribute in SearchField - not just database fields - so you could add a method that returns the year, and use that:
#register_snippet
class EventSnippet(index.Indexed, Event):
# ...
def get_year(self):
return self.day.calendar_year
search_fields = [
index.SearchField('get_year', partial_match=True),
index.SearchField('name', partial_match=True),
]