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

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)
     """