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``PiccoloCRUD`` now handles foreign key violation errors gracefully.
For example, if we have tables like this:
.. code-block:: python
class Director(Table):
name = Varchar()
class Movie(Table):
name = Varchar()
director = ForeignKey(Director, on_delete=OnDelete.restrict)
The ``ON DELETE RESTRICT`` constraint means we're not allowed to delete a
director if a movie has a foreign key to it.
We now get a ``422`` error response, with an error message which we can display
in Piccolo Admin.
Support for Python 3.7 has also been dropped, as it's end of life.
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