"Unfollow" (unsubscribe) feature

Hello I am having trouble implementing the last bit of code before I can (fingers crossed) launch my page. I implemented a "follow" feature (button) where a user can click to follow a story. All stories are displayed on the browse page but once followed, the stories display on the "library page". I got this to work however, I am having trouble with the "unfollow" portion to remove it from the library. Would ideally be the same button toggling between follow/unfollow, and so a conditional.

These are my routes:

// INDEX - show all stories
router.get("/browse", function(req, res){
    // Get all stories from DB
    Story.find({}, function(err, allStories){
        if(err){
            console.log(err)
        } else {
            res.render("stories/index", {stories:allStories})
        }
    })
})

// LIBRARY - show all stories in Library

router.get("/stories/:id/follow", middleware.isLoggedIn, function(req, res){
    User.findById(req.user.id, function(err, foundUser){
        if(err){
            console.log(err)
        } else{
            foundUser.subscriptions.push(req.params.id);
            foundUser.save();
            res.redirect("/library")
        }
    });
});

router.get("/library", middleware.isLoggedIn, function(req, res){
    User.findById(req.user.id).populate("subscriptions").exec(function(err, foundUser){
        if(err){
            console.log(err)
        } else {
            res.render("library", {subscriptions: foundUser.subscriptions});
        }
    });
});

Schemas:

var UserSchema = new mongoose.Schema({
    username: String,
    password: String,
    subscriptions: [
      {
         type: mongoose.Schema.Types.ObjectId,
         ref: "Story"
      }
  ]
})

var storySchema = new mongoose.Schema({ title: String, image: String, description: String, author: { id: { type: mongoose.Schema.Types.ObjectId, ref: "User" }, username: String }, comments:[ { type: mongoose.Schema.Types.ObjectId, ref: "Comment" } ], content:[ { type: mongoose.Schema.Types.ObjectId, ref: "Content" } ]

})var storySchema = new mongoose.Schema({
    title: String,
    image: String,
    description: String,
    author: {
        id: {
            type: mongoose.Schema.Types.ObjectId,
            ref: "User"
        },
        username: String
    },
    comments:[
        {
            type: mongoose.Schema.Types.ObjectId,
            ref: "Comment"
        }
        ],
    content:[
        {
            type: mongoose.Schema.Types.ObjectId,
            ref: "Content"
        }
        ]
})

Looking for something like ""if user subscriptions array already contains story id, then remove it, otherwise add it". Any help would be of tremendous value! Thank You.

via nguer092