Improve expression support for __search lookup and SearchQuery

[Baptiste Mispelon]

(not sure whether to categorize this as a bug or a new feature)

I've been trying to implement some kind of reverse full text search where I store keywords in the database and I want to query models whose keywords would match a given piece of text.

Here's a simplified model of what I'm working with:

class SavedSearch(models.Model):
    keywords = models.TextField()

    def __str__(self):
        return self.keywords

I've managed to achieve what I want in the case of the default search configuration using annotation and wrapping things with Value or Cast:

# This works
search_query = Cast('keywords', output_field=SearchQueryField())
search_vector = SearchVector(Value("lorem ipsum ...", output_field=TextField()))
qs = SavedSearch.objects.annotate(search=search_vector).filter(search=search_query)

But if I want to use a custom search configuration, things don't work anymore:

# This doesn't work (can't adapt type 'F')
search_query = SearchQuery(F('keywords'), config='english', search_type='plain')
search_vector = SearchVector(Value("lorem ipsum ...", output_field=TextField()))
qs = SavedSearch.objects.annotate(search=search_vector).filter(search=search_query)

I'm not very familiar with the inner workings of Lookup objects but I did some digging and I think I came up with a fix which involved fixing two separate issues:

1) SearchQuery doesn't currently support anyting other than plain values (str). Fixing this required changing both resolve_expression() and as_sql().
2) the __search lookup doesn't support things like F objects because of it assumes that any value with a resolve_expression method must be a SearchQuery object.

[Baptiste Mispelon]

Pull request here: ​https://github.com/django/django/pull/12525

[Mariusz Felisiak <felisiak.mariusz@…>]

In dd704c6:

Refs #31340 -- Simplified SearchQuery by making it subclass Func.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 3baf92cf:

Fixed #31340 -- Allowed query expressions in SearchQuery.value and search lookup.

[David]

Does the current implementation works well with custom SQL functions which returns tsquery and does not use any field which is already a tsquery?

In one of my project I have created a custom SQL function which takes a list of integers (the IDs of some row in a specific table) and builds a tsquery. I tought that using the following it would work well while it raises an exception

class TheModel(models.Model):
   searchable = SearchVectorField()


TheModel.objects.filter(searchable=models.Func(123, 345, function='my_tsquery_func', output_field=SearchQueryField())
# django.db.utils.ProgrammingError: function plainto_tsquery(tsquery) does not exist

Because the SQL produced is and this produces an exception.

SELECT id, searchable
FROM myapp_themodel
WHERE searchable @@ plainto_tsquery(my_tsquery_func(123, 345));

To add more context the my_tsquery_func looks like the following (the goal is to combine tsvectors into a tsquery to build a similarity algorithm):

CREATE OR REPLACE FUNCTION my_tsquery_func(first_id bigint, second_id bigint) RETURNS tsquery AS $$
SELECT
    to_tsquery(
        array_to_string(
            tsvector_to_array(
                (SELECT  searchable FROM myapp_themodel WHERE id = first_id LIMIT 1)
               || (SELECT  searchable FROM myapp_themodel WHERE id = second_id LIMIT 1)
            ),
            ' | '
        )
    );
$$ LANGUAGE sql PARALLEL SAFE STABLE;

[Simon Charette]

I think the patch should have checked output_field instead of special casing SearchQuery and CombinedSearchQuery in this form

django/contrib/postgres/search.py

@@ -16 +16 @@
     lookup_name = "exact"
 
     def process_rhs(self, qn, connection):
-        if not isinstance(self.rhs, (SearchQuery, CombinedSearchQuery)):
+        if not isinstance(
+            getattr(self.rhs, "_output_field_or_none", None),
+            SearchQueryField,
+        ):
             config = getattr(self.lhs, "config", None)
             self.rhs = SearchQuery(self.rhs, config=config)
         rhs, rhs_params = super().process_rhs(qn, connection)
@@ -240 +243 @@
         return "(%s)" % super().__str__()
 
 
+register_combinable_fields(
+    SearchQueryField, SearchQueryCombinable.BITAND, SearchQueryField, SearchQueryField
+)
+
+register_combinable_fields(
+    SearchQueryField, SearchQueryCombinable.BITOR, SearchQueryField, SearchQueryField
+)
+
+
 class SearchRank(Func):
     function = "ts_rank"
     output_field = FloatField()

as in its current form it disallows usage of Func(output_field=SearchQueryField()).

Happy to accept a new ticket with the above if you're willing to write tests, in the mean time your best bet is likely to subclass SearchQuery and override __init__ to assign self.function = "my_tsquery_func" instead and support a different signature.