> Immutable = require('immutable')
> Model = Immutable.Record({})
[Function: Record]
> Model.prototype.toJSON = function() { return 'model' }
[Function]
> JSON.stringify({ a: new Model() })
'{"a":"model"}'
> JSON.stringify(Immutable.Map({ a: new Model() }))
'{"a":{}}'

I expected the last line to return '{"a":"model"}'.

I know toJSON on iterables is an alias to toJS and calls toJS recursively which is why the Model's toJSON isn't called at all. Perhaps toJSON should instead call toJSON recursively which gives us a more familiar way to override the serialization behavior?