The documentation claims Model.setRelated and Model.appendRelated return the input model for chaining purposes:

• Model.setRelated
• Model.appendRelated

I found that these functions return undefined instead of model as the documentation says they should. Opening up the source code it looks like these functions are incorrectly returning this (which is undefined in the scope of the function) instead of model.

The offending lines:

• https://github.com/Vincit/objection.js/blame/master/lib/model/modelSet.js#L98
• https://github.com/Vincit/objection.js/blame/master/lib/model/modelSet.js#L114

It looks like changing these lines to return model should fix the issue.