Custom setter on MixedType doesn't know path when setting nested property

This is a follow on related to #1418 (Setting nested path into attribute of Types.Mixed is ignored).

I have a MixedType property with a customer setter defined. When setting a nested property of that MixedType property, the setter is passed the value being set but doesn't receive any information about the path being set.

This gist demonstrates: https://gist.github.com/asalant/5423400

The reason this is an issue is that if I want to do any preprocessing of the set value, I need to know the path for the value - is it resetting the whole MixedType or setting a child property? It's interesting that the second argument appears to be the schema definition for the property.

Looking at mongoose source I did not see an obvious way to get this value to the setter.

Alon

(https://groups.google.com/forum/?fromgroups=#!topic/mongoose-orm/Vo2HdUAGS6U)

Start with the linked gist and trace the MixedType custom-setter handling in the Mongoose source, comparing whole-property assignment with nested-property assignment. Confirm how the setter arguments differ and determine where the nested path is lost. Done means a custom setter can distinguish those cases and the demonstrated behavior is covered by a regression test.

Written by the indexing model from the issue text.