Add test for inserting a dollar prefixed string

This paves the way for fixing a problem with update in INTPYTHON-827.

Author: WaVEV

django_mongodb_backend/test.py

@@ -5,6 +5,7 @@
 
 class MongoTestCaseMixin:
     maxDiff = None
+    query_types = {"SON": SON, "ObjectId": ObjectId, "Decimal128": Decimal128}
 
     def assertAggregateQuery(self, query, expected_collection, expected_pipeline):
         """
@@ -15,7 +16,15 @@ def assertAggregateQuery(self, query, expected_collection, expected_pipeline):
         _, collection, operator = prefix.split(".")
         self.assertEqual(operator, "aggregate")
         self.assertEqual(collection, expected_collection)
-        self.assertEqual(
-            eval(pipeline[:-1], {"SON": SON, "ObjectId": ObjectId, "Decimal128": Decimal128}, {}),  # noqa: S307
-            expected_pipeline,
-        )
+        self.assertEqual(eval(pipeline[:-1], self.query_types, {}), expected_pipeline)  # noqa: S307
+
+    def assertInsertQuery(self, query, expected_collection, expected_documents):
+        """
+        Assert that the logged query is equal to:
+            db.{expected_collection}.insert_many({expected_documents})
+        """
+        prefix, pipeline = query.split("(", 1)
+        _, collection, operator = prefix.split(".")
+        self.assertEqual(operator, "insert_many")
+        self.assertEqual(collection, expected_collection)
+        self.assertEqual(eval(pipeline[:-1], self.query_types), expected_documents)  # noqa: S307

tests/basic_/__init__.py

@@ -0,0 +1,8 @@
+from django.db import models
+
+
+class Author(models.Model):
+    name = models.CharField(max_length=50)
+
+    def __str__(self):
+        return self.name

tests/basic_/models.py

@@ -0,0 +1,20 @@
+"""Literals that MongoDB intreprets as expressions are escaped."""
+
+from django.test import TestCase
+
+from django_mongodb_backend.test import MongoTestCaseMixin
+
+from .models import Author
+
+
+class ModelCreationTests(MongoTestCaseMixin, TestCase):
+    def test_dollar_prefixed_string(self):
+        # No escaping is needed because MongoDB's insert doesn't treat
+        # dollar-prefixed strings as expressions.
+        with self.assertNumQueries(1) as ctx:
+            obj = Author.objects.create(name="$foobar")
+        obj.refresh_from_db()
+        self.assertEqual(obj.name, "$foobar")
+        self.assertInsertQuery(
+            ctx.captured_queries[0]["sql"], "basic__author", [{"name": "$foobar"}]
+        )