Arango-orm

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- Documentation updates and dedicated documentation site at: https://arango-orm.readthedocs.io/en/latest/index.html
- Graph.delete_tree method to delete an already "expanded" object and all it's linked data.
- Fixes memory leak related to marshmallow schemas not getting cleaned up correctly. Thanks SoundsSerious

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Merging PR from chris-green with thanks.

- Return embedded objects in schemas (using Nested fields) as their object equivalent instead of dicts.
This change instanciates all objects upon load using the post_load decorator upon schemas,
automatically instanciating both top level objects and their embedded counterparts.

- Add docker-compose and pytest for quick, consistent setup of application and tests for contributors to package.

- Added getting-started to explain how to get started on how to run tests for package.

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- Support for specifiying a normal name for the `_key` field (only for models, for querying etc still need to use `_key`)

.. code-block:: python

class Person(Collection):

__collection__ = "persons"

_key_field = 'name'

name = String(required=True, allow_none=False)
age = Integer(allow_none=True, missing=None)

p1 = Person(_key='abc', age=30)
print(p1._key)
print(p1.name) same as p1._key
print(p1.age)

p2 = Person()
p2.name = 'xyz'
print(p2._key)
print(p2.name) same as p2._key

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- Code updated to work with latest marshmallow (3.7.0).

- Validation is done on record add and update time too. Fixes 70. Previously
data was added without validation and when it was read back, validation was done
at that time (default marshmallow behavior) and caused validation errors.

- [Backward incompatible] Setting _key field to require=True means that it's value
should always be provided and is not to be auto-generated. To keep the previous
behavior do not set `required=True` for the field. This will allow both setting
it or having it auto generated.

.. code-block:: python

allows _key auto-generation
class Person(Collection):

__collection__ = "persons"

_key = String()
name = String(required=True, allow_none=False)
age = Integer(allow_none=True, missing=None)
dob = Date(allow_none=True, missing=None)

This will not allow _key auto-generation
class Person(Collection):

__collection__ = "persons"

_key = String(required=True)
name = String(required=True, allow_none=False)
age = Integer(allow_none=True, missing=None)
dob = Date(allow_none=True, missing=None)

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- Support for specifying cursor ttl for queries. Otherwise arangodb has a small delay in returning records after record number 1000 which causes no cursor errors. Thanks wonderbeyond for the PR.

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- Bug fix for https://github.com/threatify/arango-orm/issues/55