def update(cls, dframe, dataset):
"""
Update *dataset* by overwriting all observations with the given
*dframe*.
"""
previous_dtypes = cls.find(dataset, as_df=True).dtypes.to_dict()
new_dtypes = dframe.dtypes.to_dict().items()
cols_to_add = dict([(name, dtype) for name, dtype in
new_dtypes if name not in previous_dtypes])
Dataset.update_schema(dataset, cols_to_add)
cls.delete(dataset)
cls.save(dframe, dataset)
return cls.find(dataset, as_df=True)
