`BaseDocument.to_dict` serializes a document/embedded document into a dict,
which can be easily consumed by other modules (which in this case don't need
to be aware of MongoEngine-specific object types).
The output dict contains key-value pairs where:
* Keys are field names as they're defined on the document (as opposed to e.g.
how they're stored in MongoDB).
* Values are field values in their deserialized form (i.e. the same form that
you get when you access `doc.some_field_name`).
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