Redis-astra

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- Auto import foreign models

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- Allow use '0' or 0 as object's pk
- defaultPk attribute for ForeignKey provide default object for not
assigned foreign object. This feature provide access to deep properties
without risk of catch AttributeError. Eg: o = user.site.owner, where site
and owner is ForeignKey to other model, but site is not set. In simple case
we're catch AttributeError: 'NoneType' object has no attribute 'owner'. But,
when site is ForeignKey(to='Site', defaultPk=0), then site always
present as 'Site' model. At the moment, you need provide your 'Site'
behavior with pk key is 0.
- Performance improvement: models using lazy initialization now

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- (!) Deprecated type: ForeignKeyHash. Link now must be ForeignKey
- Support "withscores" in zrange, zrangebyscore, zrevrange, zrevrangebyscore
methods. This methods return list of tuples

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- Minor fixes: support ttl, expire commands for scalar fields

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- List, Set and SortedSet not necessarily refer to other models
- Integer and Char fields return 0 and '' instead None if not assigned
- Enum must have default value