Schema.isModified not always accurate for subschema paths

[quinnlas]

Do you want to request a feature or report a bug?
bug

What is the current behavior?
With a schema like so:

const secondLevelSchema = new Schema({
  id: Number,
  text: String
})

const topLevelSchema = new Schema({
  secondLevel: {
    type: secondLevelSchema
  }
}

If you create a new topLevelItem and set one of the properties of secondLevel but not the other,
for instance:
new TopLevelModel({ secondLevel: { text: 'example' } })

then you will have the following strange behavior for the other property:

topLevelItem.isModified('secondLevel.id') //true
topLevelItem.secondLevel.isModified('id')) //false

What is the expected behavior?
both values should be false, since the id has not been modified

[vkarpov15]

We fixed it so that both topLevelItem.isModified('secondLevel.id') and topLevelItem.secondLevel.isModified('id')) return true. The reason this behavior is that new TopLevelModel({ secondLevel: { text: 'example' } }) sets secondLevel, so secondLevel is modified and therefore all of its children are modified as well. This is because isModified() exists primarily to help save() figure out what paths were updated.