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

i want generate slug when update called in mongoose, have , works.

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

the problem slug unique: true field if add 2 posts same title error, example this:

"slug: post-2\" exists." 

so before calling next() want sure if slug exists calling find, have 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 called following error:

          ? callback(null, docs)             ^  typeerror: callback not function 

how should continue?

try this

// uniq slug generator function, converts `text` `text1` if `text` exists  schema.statics.generateslug = function(text, suffix, callback){     var _this = this;     var possibletext = text + (suffix || '');     _this.findone({         slug: slugify(possibletext.tolowercase())     }, function(err, item){         if(!err) {             if(!item) {                 callback(possibletext);             } else {                 return _this.generateslug(text, (suffix || 0) + 1, callback);             }         } else {             callback(null);         }     }); };  schema.pre('update', function (next) {     var _this = this;     const title = _this._update.$set['title'];     _this.generateslug(title, null, function(possibleslug) {         if (possibleslug) {              _this._update.$set['slug'] = possibleslug;              next();         } else return next(new error('no possible slug'));      }); })