Virtual field in child is undefined in toJSON from parent

[DenMesh]

Do you want to request a feature or report a bug?
Report a bug.
What is the current behavior?
See comments on two last lines of code - getter is skipped when calling toJSON on parent, despite having "getters: true" on all schemas.
If the current behavior is a bug, please provide the steps to reproduce.

var { model, Schema } = require('mongoose');

var childSchema = new Schema({}, {
	toJSON: { getters: true }
});

childSchema.virtual('field')
	.get(function(){
		return this._field;
	})
	.set(function(v){
		return this._field = v;
	});

var Child = model('Child', childSchema);

var parentSchema = new Schema({
	child: { type: Schema.Types.ObjectId, ref: 'Child', get: childGetter }
}, {
	toJSON: { getters: true }
});

function childGetter(child){
	child.field = true;
	return child;
}

var Parent = model('Parent', parentSchema);

//=====================
var child = new Child();
var parent = new Parent({ child });

console.log(parent.toJSON().child.field); //prints 'undefined'
console.log(parent.child.toJSON().field); //prints 'true'

What is the expected behavior?
To have "true" as output of both console.logs.
What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node.js 8.9.3
Mongoose 5.6.0
MongoDB irrelevant - MWE does not connect to DB at all

[DenMesh]

Upd: if you access the virtual prior to toJSONing it, everything works fine:

var child = new Child();
var parent = new Parent({ child });

console.log(parent.child.field); //prints 'true'
console.log(parent.toJSON().child.field); //prints 'true'
console.log(parent.child.toJSON().field); //prints 'true'

[vkarpov15]

Confirmed this is a bug, looks related to the work from #7442. I tried a few different ways to fix it, but having some trouble getting the tests for this issue, #7442, and #1376 to pass at the same time. Will try again later 👍