Mongoengine

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- Replace deprecated PyMongo v2.x methods with their v3.x equivalents in the ``SequenceField``. 2097
- Various code clarity and documentation improvements.

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- Fix a bug introduced in 0.18.0 which was causing ``Document.save`` to update all the fields instead of updating only the modified fields. This bug only occurred when using a custom PK. 2082
- Add Python 3.7 to Travis CI. 2058

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- Drop support for EOL'd MongoDB v2.6, v3.0, and v3.2.
- MongoEngine now requires PyMongo >= v3.4. Travis CI now tests against MongoDB v3.4 – v3.6 and PyMongo v3.4 – v3.6. 2017 2066
- Improve performance by avoiding a call to ``to_mongo`` in ``Document.save``. 2049
- Connection/disconnection improvements:
- Expose ``mongoengine.connection.disconnect`` and ``mongoengine.connection.disconnect_all``.
- Fix disconnecting. 566 1599 605 607 1213 565
- Improve documentation of ``connect``/``disconnect``.
- Fix issue when using multiple connections to the same mongo with different credentials. 2047
- ``connect`` fails immediately when db name contains invalid characters. 2031 1718
- Fix the default write concern of ``Document.save`` that was overwriting the connection write concern. 568
- Fix querying on ``List(EmbeddedDocument)`` subclasses fields. 1961 1492
- Fix querying on ``(Generic)EmbeddedDocument`` subclasses fields. 475
- Fix ``QuerySet.aggregate`` so that it takes limit and skip value into account. 2029
- Generate unique indices for ``SortedListField`` and ``EmbeddedDocumentListFields``. 2020
- BREAKING CHANGE: Changed the behavior of a custom field validator (i.e ``validation`` parameter of a ``Field``). It is now expected to raise a ``ValidationError`` instead of returning ``True``/``False``. 2050
- BREAKING CHANGES (associated with connection/disconnection fixes):
- Calling ``connect`` 2 times with the same alias and different parameter will raise an error (should call ``disconnect`` first).
- ``disconnect`` now clears ``mongoengine.connection._connection_settings``.
- ``disconnect`` now clears the cached attribute ``Document._collection``.
- BREAKING CHANGE: ``EmbeddedDocument.save`` & ``.reload`` no longer exist. 1552

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- POTENTIAL BREAKING CHANGE: All result fields are now passed, including internal fields (``_cls``, ``_id``) when using ``QuerySet.as_pymongo``. 1976
- Document a BREAKING CHANGE introduced in 0.15.3 and not reported at that time. 1995
- DEPRECATION: ``EmbeddedDocument.save`` & ``.reload`` are marked as deprecated and will be removed in a next version of MongoEngine. 1552
- Fix ``QuerySet.only`` working improperly after using ``QuerySet.count`` of the same instance of a ``QuerySet``.
- Fix ``batch_size`` that was not copied when cloning a ``QuerySet`` object. 2011
- Fix ``InvalidStringData`` error when using ``modify`` on a ``BinaryField``. 1127
- Fix test suite and CI to support MongoDB v3.4. 1445
- Fix reference fields querying the database on each access if value contains orphan DBRefs.

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- Fix ``$push`` with the ``$position`` operator not working with lists in embedded documents. 1965

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- Fix ``Document.save`` that fails when called with ``write_concern=None`` (regression of 0.16.1). 1958