Grid Hierarchy - sub grid saving records error handling

[Jonathan]

I've followed code from the Grid Hierarchy demo to create a sub grid and have made it editable.

I have got validation and record editing working fine.

However I am struggling with creating new records. I have a constraint on the SQL table to disallow duplicate values (combination of 2 columns).

However I cannot seem to handle this error. Currently the UI adds the record, but doesn't create it in SQL.

I also noticed I cannot use decorate my controller action with HTTP post, as this yields a 404 error. Perhaps this is due to the sub grid.

Are there any examples there of a 2 level hierarchy grid with CRUD operations on the sub grid with error handling please?

EDIT: Does anyone know what this symbol means? I get this after trying to create a new record in the sub grid.