Add two fields to NeutronDbObjectDuplicateEntry

When people were handling duplicated entry error from DB, sometimes
they were trying to find out the specific columns where the value is
duplicated. However, such information are not in the exception object
which is inconvenient. People were even trying to do another query to
find out the duplicated columns. This patch addresses it.

Change-Id: If3ebcdfb5c3a5ddba1e2ebb6cb0ff79239e1ccd3

neutron_lib/objects/exceptions.py

@@ -21,6 +21,18 @@ class NeutronObjectUpdateForbidden(exceptions.NeutronException):
 
 
 class NeutronDbObjectDuplicateEntry(exceptions.Conflict):
+    """Duplicate entry at unique column error.
+
+    Raised when made an attempt to write to a unique column the same entry as
+    existing one. :attr: `columns` available on an instance of the exception
+    and could be used at error handling::
+
+       try:
+           obj_ref.save()
+       except NeutronDbObjectDuplicateEntry as e:
+           if 'colname' in e.columns:
+               # Handle error.
+    """
     message = _("Failed to create a duplicate %(object_type)s: "
                 "for attribute(s) %(attributes)s with value(s) %(values)s")
 
@@ -30,6 +42,8 @@ class NeutronDbObjectDuplicateEntry(exceptions.Conflict):
                                                   fully_qualified=False),
             attributes=db_exception.columns,
             values=db_exception.value)
+        self.columns = db_exception.columns
+        self.value = db_exception.value
 
 
 class NeutronDbObjectNotFoundByModel(exceptions.NotFound):

releasenotes/notes/add-two-fields-to-duplicated-entry-exception-75b0e07c6e1cc6ae.yaml

@@ -0,0 +1,6 @@
+---
+other:
+  - |
+    Add two fields ``columns`` and ``value`` to exception
+    ``NeutronDbObjectDuplicateEntry``. These two fields are populated
+    from the corresponding db exception.