Tangentially related to issue: https://jira.mongodb.org/browse/MONGOID-4816

Mongoose lets you optionally specify which field to use as the discriminator for subclasses, rather than just hardcoding _type.

It would be nice if Mongoid had an macro/option to do the same.

Scope of work:

Investigate how mongoose and doctrine handle overriding discriminator field name
Write a proposed design document
Adjust submitted PR or implemented design in another way as appropriate
Define test plan
Write tests to satisfy test plan
Add user documentation to tutorial