'Prepares the visible_blocks_id field for the grade record,
using the hash of the visible_blocks field.  Specifying
the hashed field eliminates extra queries to get the
VisibleBlocks record.  Use this variation of preparing
the params when you are sure of the existence of the
VisibleBlock.'

@classmethod
def _prepare_params_visible_blocks_id(cls, params):

    params['visible_blocks_id'] = params['visible_blocks'].hash_value
    del params['visible_blocks']
