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Here's mogenerator's elevator pitch:

mogenerator is a command-line tool that, given an .xcdatamodel file, will generate two classes per entity. The first class, _MyEntity, is intended solely for machine consumption and will be continuously overwritten to stay in sync with your data model. The second class, MyEntity, subclasses _MyEntity, won't ever be overwritten and is a great place to put your custom logic.

Want more detail? John Blanco has authored a detailed writeup about mogenerator.

Senseful wrote up a nice summary of mogenerator's command-line options.

• [NEW] Support for URL and UUID property types (Trevor Squires, original PR)

• [NEW] Add support for "Uses Scalar Type" (Rok Gregorič, original PR)

• [NEW] Add Swift generic fetchRequest() to generated code (0xpablo, original PR)

• [NEW] Add nullability annotations for generated primitive accessors of optional, to-one relationships. (Michael Babin, original PR)

• [NEW] Expose allAttributes and allRelationships (Trevor Squires, original PR)

• [NEW] Add template booleans to identify custom attribute type use. (Aleksandar Vacić, original PR)

• [NEW] Support "Custom Class" for Transformable attributes (Tomek Cejner, commit)

• [NEW] Add parameter to ignore entities (Martin Kim Dung-Pham, original PR)

• [CHANGE] Use Data instead of NSData with Swift (Christopher Rogers, original PR)

• [CHANGE] Dropped .pkg binary releases. mac OS 10.14 Mojave won't install unsigned pkgs by default any more, and I'm not paying Apple $100/year so I can distribute open source binaries. Use Homebrew to install and update mogenerator binaries. (rentzsch)

• [CHANGE] Travis updates (Trevor Squires, commit, commit)

• [FIX] Correct Swift machine template for singleton (fetchOne…) fetch request results. (Warren Burton, original PR)

Further history is listed in the Version History file.