introduce ActiveRef and TranslatedRef lookups

localized_fields/lookups.py

@@ -5,7 +5,9 @@ from django.contrib.postgres.lookups import (
     TrigramSimilar,
     Unaccent,
 )
-from django.db.models.expressions import Col
+from django.db.models import TextField, Transform
+from django.db.models.expressions import Col, Func, Value
+from django.db.models.functions import Coalesce
 from django.db.models.lookups import (
     Contains,
     EndsWith,
@@ -22,6 +24,16 @@ from django.db.models.lookups import (
 )
 from django.utils import translation
 
+from .fields import LocalizedField
+
+try:
+    from django.db.models.functions import NullIf
+except ImportError:
+    # for Django < 2.2
+    class NullIf(Func):
+        function = "NULLIF"
+        arity = 2
+
 
 class LocalizedLookupMixin:
     def process_lhs(self, qn, connection):
@@ -92,3 +104,43 @@ class LocalizedRegexWith(LocalizedLookupMixin, Regex):
 
 class LocalizedIRegexWith(LocalizedLookupMixin, IRegex):
     pass
+
+
+@LocalizedField.register_lookup
+class ActiveRefLookup(Transform):
+    output_field = TextField()
+    lookup_name = "active_ref"
+    arity = None
+
+    def as_sql(self, compiler, connection):
+        language = translation.get_language() or settings.LANGUAGE_CODE
+        return KeyTransform(language, self.lhs).as_sql(compiler, connection)
+
+
+@LocalizedField.register_lookup
+class TranslatedRefLookup(Transform):
+    output_field = TextField()
+    lookup_name = "translated_ref"
+    arity = None
+
+    def as_sql(self, compiler, connection):
+        language = translation.get_language()
+        fallback_config = getattr(settings, "LOCALIZED_FIELDS_FALLBACKS", {})
+        target_languages = fallback_config.get(language, [])
+        if not target_languages and language != settings.LANGUAGE_CODE:
+            target_languages.append(settings.LANGUAGE_CODE)
+
+        if language:
+            target_languages.insert(0, language)
+
+        if len(target_languages) > 1:
+            return Coalesce(
+                *[
+                    NullIf(KeyTransform(language, self.lhs), Value(""))
+                    for language in target_languages
+                ]
+            ).as_sql(compiler, connection)
+
+        return KeyTransform(target_languages[0], self.lhs).as_sql(
+            compiler, connection
+        )