deletePost: (req, res) => {
        "use strict";
        let id = req.params.id;

        User.findOneAndRemove({_id: id}).then(user => {
            user.prepareDelete();
            res.redirect('/admin/user/all');
        })
    }
----------------
//user.prepareDelete
prepareDelete: function () {
        let Comment = mongoose.model('Comment')
        for (let comment of this.comments) {
            Comment.findById(comment).then(comment => {
                comment.prepareDelete()
                comment.remove()
            })
        }

        let Problems = mongoose.model('Problems')
        for (let problem of this.problems) {
            Problems.findById(problem).then(problem => {
                "use strict";
                problem.prepareDelete()
                problem.remove()
            })
        }

    }
---------------
//comment.prepareDelete
prepareDelete: function () {
        let User = mongoose.model('User')
        User.findById(this.author).then(user => {
            "use strict";
            if (user) {
                user.comments.remove(this.id)
                user.save()
            }
        })

        let Problems = mongoose.model('Problems')
        Problems.findById(this.target).then(problem => {
            "use strict";
            if (problem) {
                problem.comments.remove(this.id)
                problem.save()
            }
        })
    }
-------------------
//problem.prepareDelete
prepareDelete: function () {
        let User = mongoose.model('User')
        User.findById(this.author).then(user => {
            "use strict";
            if (user) {
                user.problems.remove(this.id)
                user.save()
            }
        })

        let Comment = mongoose.model('Comment')
        for (let commentId of this.comments) {
            Comment.findOneAndRemove(commentId).populate('author').then(comment => {
                "use strict";
                if (comment) {
                    let author = comment.author
                    let index = author.comments.indexOf(commentId)

                    let count = 1
                    author.comments.splice(index, count);
                    author.save()
                    comment.save()
                }
            })
        }
    }