remove unnecessary $convert in ObjectIdAutoField $lookup queries

django_mongodb/fields/auto.py

@@ -34,6 +34,9 @@ def get_prep_value(self, value):
     def db_type(self, connection):
         return "objectId"
 
+    def rel_db_type(self, connection):
+        return "objectId"
+
     def to_python(self, value):
         if value is None or isinstance(value, int):
             return value

tests/queries_/models.py

@@ -0,0 +1,16 @@
+from django.db import models
+
+
+class Author(models.Model):
+    name = models.CharField(max_length=10)
+
+    def __str__(self):
+        return self.name
+
+
+class Book(models.Model):
+    title = models.CharField(max_length=10)
+    author = models.ForeignKey(Author, models.CASCADE)
+
+    def __str__(self):
+        return self.title

tests/queries_/test_mql.py

@@ -0,0 +1,26 @@
+from django.test import TestCase
+
+from .models import Author, Book
+
+
+class MQLTests(TestCase):
+    def test_all(self):
+        with self.assertNumQueries(1) as ctx:
+            list(Author.objects.all())
+        query = ctx.captured_queries[0]["sql"]
+        self.assertEqual(query, "db.queries__author.aggregate([{'$match': {'$expr': {}}}])")
+
+    def test_join(self):
+        with self.assertNumQueries(1) as ctx:
+            list(Book.objects.filter(author__name="Bob"))
+        query = ctx.captured_queries[0]["sql"]
+        self.assertEqual(
+            query,
+            "db.queries__book.aggregate(["
+            "{'$lookup': {'from': 'queries__author', "
+            "'let': {'parent__field__0': '$author_id'}, "
+            "'pipeline': [{'$match': {'$expr': "
+            "{'$and': [{'$eq': ['$$parent__field__0', '$_id']}]}}}], 'as': 'queries__author'}}, "
+            "{'$unwind': '$queries__author'}, "
+            "{'$match': {'$expr': {'$eq': ['$queries__author.name', 'Bob']}}}])",
+        )