无法读取未定义的push属性？即使我将评论推送到评论数组，为什么仍会收到此错误？

router.post("/", middleware.isLoggedIn, function(req, res) {
  arms.findById(req.params.id, function(err, armsFound) {
    if (err) {
      console.log(err);
      res.redirect("back");
    } else {
      comments.create(req.body.comment, function(err, comment) {
        if (err) {
          console.log(err);
          res.redirect("back");
        } else {
          comment.author.id = req.user._id;
          comment.author.username = req.user.username;
          comment.save();
          arms.comments.push(comment);
          arms.save();
          req.flash("success", "Successfully added comment");
          res.redirect("/arms/" + arms._id);
        }
      });
    }
  });
});

const mongoose = require("mongoose");

const armSchema = new mongoose.Schema({
  name: {
    type: String,
    required: true
  },
  price: String,
  image: String,
  description: String,
  owner: {
    id: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "User"
    },
    username: String,
  },
  comments: [{
    type: mongoose.Schema.Types.ObjectId,
    ref: "Comment"
  }]
})

module.exports = mongoose.model("arms", armSchema);

const mongoose = require("mongoose");

const commentSchema = new mongoose.Schema({
  text: String,
  author: {
    id: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "User"
    },
    username: String
  }
})

module.exports = mongoose.model("Comment", commentSchema);