Tortoise-orm

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- ``iexact`` filter modifier was implemented. Queries like ``«queryset».filter(name__iexact=...)`` will perform case-insensitive search.

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- Fix minor bug in ``Model.__init__`` where we raise the wrong error on setting RFK/M2M values directly.
- Fields in ``Queryset.values_list()`` is now in the defined Model order.
- Fields in ``Queryset.values()`` is now in the defined Model order.

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- Sample Starlette integration
- Relational fields are now lazily constructed via properties instead of in the constructor,
this results in a significant overhead reduction for Model instantiation with many relationships.

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- Assigning to the FK field will correctly set the associated db-field
- Reading a nullalble FK field can now be None
- Nullalble FK fields reverse-FK is now also nullable
- Deleting a nullable FK field sets it to None

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- Fixed installing Tortoise-ORM in non-unicode systems. (180)
- ``«queryset».update(…)`` now correctly uses the DB-specific ``to_db_value()``
- ``fetch_related(…)`` now correctly encodes non-integer keys.
- ``ForeignKey`` fields of type ``UUIDField`` are now escaped consistently.
- Pre-generated ForeignKey fields (e.g. UUIDField) is now checked for persistence correctly.
- Duplicate M2M ``.add(…)`` now checks using consistent field encoding.
- ``source_field`` Fields are now handled correctly for ordering.
- ``source_field`` Fields are now handled correctly for updating.

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* Security fixes for ``«model».save()`` & ``«model».delete()``:

This is now fully parametrized, and these operations are no longer susceptible to escaping issues.

* Performance improvements:

- Simple update is now ~3-6× faster
- Partial update is now ~3× faster
- Delete is now ~2.7x faster

- Fix generated Schema Primary Key for ``BigIntField`` for MySQL and PostgreSQL.
- Added support for using a ``SmallIntField`` as a auto-gen Primary Key.
- Ensure that default PK is added to the top of the attrs.