Generate a unique slug on mongoose schema.pre.('update')

I want to generate an slug when update is called in mongoose, now I have this and it works.

schema.pre('update', function (next) {
    const title = this._update.$set['title']
    this._update.$set['slug'] = slugify(title.toLowerCase())
    next()
})

The problem is that slug is a unique: true field so if add two posts with the same title I get an error, for example like this:

"slug: post-2\" already exists."

So now before calling next() I want to be sure if my slug already exists calling find, now I have this code:

.pre('update', function (next) {
    const title = this._update.$set['title']
    this._update.$set['slug'] = slugify(title.toLowerCase())

    this._find({ slug }, function (err, docs) {
        if (err)
            console.log(err)
        else
            console.log(docs)
        next()
    })
})

But when is called I get the following error:

          ? callback(null, docs)
            ^

TypeError: callback is not a function

How should I continue?

via user3561335