Fix bug introduced in -1.19 related to DictField validate failing without default connection

docs/changelog.rst

@@ -7,6 +7,11 @@ Development
 ===========
 - (Fill this out as you fix issues and develop your features).
 
+Changes in 0.19.2
+=================
+- DictField validate failed without default connection (bug introduced in 0.19.0) #2239
+
+
 Changes in 0.19.1
 =================
 - Requires Pillow < 7.0.0 as it dropped Python2 support

mongoengine/fields.py

@@ -1088,14 +1088,12 @@ class DictField(ComplexBaseField):
             msg = "Invalid dictionary key - documents must have only string keys"
             self.error(msg)
 
-        curr_mongo_ver = get_mongodb_version()
-
-        if curr_mongo_ver < MONGODB_36 and key_has_dot_or_dollar(value):
-            self.error(
-                'Invalid dictionary key name - keys may not contain "."'
-                ' or startswith "$" characters'
-            )
-        elif curr_mongo_ver >= MONGODB_36 and key_starts_with_dollar(value):
+        # Following condition applies to MongoDB >= 3.6
+        # older Mongo has stricter constraints but
+        # it will be rejected upon insertion anyway
+        # Having a validation that depends on the MongoDB version
+        # is not straightforward as the field isn't aware of the connected Mongo
+        if key_starts_with_dollar(value):
             self.error(
                 'Invalid dictionary key name - keys may not startswith "$" characters'
             )

mongoengine/mongodb_support.py

@@ -11,7 +11,7 @@ MONGODB_36 = (3, 6)
 
 
 def get_mongodb_version():
-    """Return the version of the connected mongoDB (first 2 digits)
+    """Return the version of the default connected mongoDB (first 2 digits)
 
     :return: tuple(int, int)
     """

tests/fields/test_dict_field.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 import pytest
+from bson import InvalidDocument
 
 from mongoengine import *
 from mongoengine.base import BaseDict
@@ -19,22 +20,24 @@ class TestDictField(MongoDBTestCase):
         post = BlogPost(info=info).save()
         assert get_as_pymongo(post) == {"_id": post.id, "info": info}
 
-    def test_general_things(self):
-        """Ensure that dict types work as expected."""
+    def test_validate_invalid_type(self):
+        class BlogPost(Document):
+            info = DictField()
 
+        BlogPost.drop_collection()
+
+        invalid_infos = ["my post", ["test", "test"], {1: "test"}]
+        for invalid_info in invalid_infos:
+            with pytest.raises(ValidationError):
+                BlogPost(info=invalid_info).validate()
+
+    def test_keys_with_dots_or_dollars(self):
         class BlogPost(Document):
             info = DictField()
 
         BlogPost.drop_collection()
 
         post = BlogPost()
-        post.info = "my post"
-        with pytest.raises(ValidationError):
-            post.validate()
-
-        post.info = ["test", "test"]
-        with pytest.raises(ValidationError):
-            post.validate()
 
         post.info = {"$title": "test"}
         with pytest.raises(ValidationError):
@@ -48,25 +51,34 @@ class TestDictField(MongoDBTestCase):
         with pytest.raises(ValidationError):
             post.validate()
 
-        post.info = {1: "test"}
-        with pytest.raises(ValidationError):
-            post.validate()
-
         post.info = {"nested": {"the.title": "test"}}
         if get_mongodb_version() < MONGODB_36:
-            with pytest.raises(ValidationError):
-                post.validate()
+            # MongoDB < 3.6 rejects dots
+            # To avoid checking the mongodb version from the DictField class
+            # we rely on MongoDB to reject the data during the save
+            post.validate()
+            with pytest.raises(InvalidDocument):
+                post.save()
         else:
             post.validate()
 
         post.info = {"dollar_and_dot": {"te$st.test": "test"}}
         if get_mongodb_version() < MONGODB_36:
-            with pytest.raises(ValidationError):
-                post.validate()
+            post.validate()
+            with pytest.raises(InvalidDocument):
+                post.save()
         else:
             post.validate()
 
-        post.info = {"title": "test"}
+    def test_general_things(self):
+        """Ensure that dict types work as expected."""
+
+        class BlogPost(Document):
+            info = DictField()
+
+        BlogPost.drop_collection()
+
+        post = BlogPost(info={"title": "test"})
         post.save()
 
         post = BlogPost()