[1.6.x] Fixed #22514 -- Prevented indexes on virtual fields [postgres].

Backport of 78c32f1caa from master

tests/indexes/models.py

@@ -2,10 +2,35 @@ from django.db import connection
 from django.db import models
 
 
+class CurrentTranslation(models.ForeignObject):
+    """
+    Creates virtual relation to the translation with model cache enabled.
+    """
+    # Avoid validation
+    requires_unique_target = False
+
+    def __init__(self, to, from_fields, to_fields, **kwargs):
+        # Disable reverse relation
+        kwargs['related_name'] = '+'
+        # Set unique to enable model cache.
+        kwargs['unique'] = True
+        super(CurrentTranslation, self).__init__(to, from_fields, to_fields, **kwargs)
+
+
+class ArticleTranslation(models.Model):
+
+    article = models.ForeignKey('indexes.Article')
+    language = models.CharField(max_length=10, unique=True)
+    content = models.TextField()
+
+
 class Article(models.Model):
     headline = models.CharField(max_length=100)
     pub_date = models.DateTimeField()
 
+    # Add virtual relation to the ArticleTranslation model.
+    translation = CurrentTranslation(ArticleTranslation, ['id'], ['article'])
+
     class Meta:
         index_together = [
             ["headline", "pub_date"],