Protean

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* Introduce Views (a.k.a Read Models)
* Refactor Domain Module to be leaner
* Introduce Class Factories for all Domain Elements
* Rename `impl` to `adapters`
* Add separate `ports` module with interfaces
* Remove `bounded_context` definition everywhere
* Rename `dict_repo` to `memory`
* Remove `*_repo` from DB adapter module names
* Rename `celery` and `inline` broker module names
* Rename `sendgrid` email module name

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* Add support to restrict a model to a specific database
* Add support for optional pickling of `List` fields (leverage Postgres List support)
* Add support for Overlap operator in SQLAlchemy repo for Postgresql
* Bugfix - Return keys as Strings for `Dict` field
* Add support for storing Array of dictionaries in Postgres JSON column
* Bugfix - Validate unique constraints in Dictionary repo
* Bugfix - Fix broken Entity construction from Elasticsearch model

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* Allow Postgres Models to choose b/w Pickle or Custom Types for Array and JSON
* Add support for Any operator/lookup for Postgres

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* Introduce support for Postgresql Array and JSON columns
* Allow restricting `List` datatype to specific content types

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* Log Protean exceptions to print helpful debug messages
* Support for installation of optional dependencies
* Remove support for Data Transfer Object
* Remove support for Request and Response Objects
* Bugfix - Throw Exceptions on Unit of Work commit failures
* Use `black` as code formatter, with a git pre-commit hook
* Add 3.9 to test matrix and cleanup Travis yml
* Bugfix - Fix broken Sphinx docs build

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* Bugfix 304 - Remove shadow object linkages with aggregates
* Raise InvalidDataError on invalid attributes to Commands
* Treat meta attributes like auto_fields and mandatory_fields as dicts instead of tuples
* Add support for nested serializers/schemas with Marshmallow
* Bugfix to not link shadow fields with Aggregate/Entity classes directly
* Bugfix to set initialize Shadow (Reference) and Value Object attributes correctly
* Store Reference and Value Object fields in `Entity.meta_` for later use
* Allow Subscribers and Handlers to hook into multiple Events
* Bugfix to avoid fetching child records without foreign key linkages
* Add support for Dict serialization
* Allow persisting and management of child entities via the Aggregate object
* Add support for using Celery as the background worker