Fix 'exact' lookup with an implicit subquery on Elasticsearch

wagtail/search/backends/elasticsearch7.py

@@ -5,7 +5,7 @@ from urllib.parse import urlparse
 
 from django.db import DEFAULT_DB_ALIAS, models
 from django.db.models.sql import Query
-from django.db.models.sql.constants import MULTI
+from django.db.models.sql.constants import MULTI, SINGLE
 from django.utils.crypto import get_random_string
 from elasticsearch import VERSION as ELASTICSEARCH_VERSION
 from elasticsearch import Elasticsearch, NotFoundError
@@ -505,6 +505,13 @@ class Elasticsearch7SearchQueryCompiler(BaseSearchQueryCompiler):
                     }
                 }
             else:
+                if isinstance(value, Query):
+                    db_alias = self.queryset._db or DEFAULT_DB_ALIAS
+                    value = value.get_compiler(db_alias).execute_sql(result_type=SINGLE)
+                    # The result is either a tuple with one element or None
+                    if value:
+                        value = value[0]
+
                 return {
                     "term": {
                         column_name: value,

wagtail/search/tests/test_backends.py

@@ -301,6 +301,23 @@ class BackendTests(WagtailTestUtils):
             [r.title for r in results], ["The Return of the King"]
         )
 
+    def test_filter_exact_values_list_subquery(self):
+        protagonist = (
+            models.Character.objects.filter(name="Frodo Baggins")
+            .order_by("novel_id")
+            .values_list("pk", flat=True)[:1]
+        )
+
+        results = self.backend.search(
+            MATCH_ALL,
+            models.Novel.objects.filter(protagonist_id=protagonist),
+        )
+
+        self.assertUnsortedListEqual(
+            [r.title for r in results],
+            ["The Fellowship of the Ring"],
+        )
+
     def test_filter_lt(self):
         results = self.backend.search(
             MATCH_ALL, models.Book.objects.filter(number_of_pages__lt=440)