根据mongoose文档，有3种方法可以在文档中添加自定义方法：
1.通过架构选项
1.直接将“方法”对象分配给模式
1.使用Schema.method（）帮助器
然而，经过多次尝试，我只设法让方法使用选项1工作。我很好奇为什么选项2和3不为我工作。以下是我的代码：
app.js

socket.on("message", async function (clusterData, callback) {
  console.log("socket event fired");
  const parentCluster = await Message.findById(clusterData.clusterId);
  coonsole.log(parentCluster); // exists as expected

  parentCluster.optionsMethod(); // log : "options method called" ✔
  parentCluster.objectMethod(); // error : parentCluster.objectMethod is not a function ❌
  parentCluster.helperMethod(); // error : parentCluster.helperMethod is not a function ❌
});

Message.js

import mongoose from "mongoose";

const messageSchema = new mongoose.Schema({
  mentions: [{ type: mongoose.Schema.Types.ObjectId, ref: "User" }],
  text: { type: String, trim: true },
  file: { type: String },
  dateString: { type: String, required: true },
  timestamp: { type: Number, required: true },
});

const messageClusterSchema = new mongoose.Schema(
  {
    sender: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "User",
      required: true,
    },
    channel: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "Channel",
      required: true,
    },
    group: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "Group",
      required: true,
    },
    content: [messageSchema],
    clusterTimestamp: {
      type: Number,
      required: true,
    },
  },
  {
    toObject: { virtuals: true },
    toJSON: { virtuals: true },
    methods: {
      optionsMethod() {
        console.log("options method called");
      },
    },
  }
);
messageClusterSchema.virtual("lastMessage").get(function () {
  return this.content[this.content.length - 1];
});

messageClusterSchema.pre("validate", function () {
  console.log("pre validate ran");
  this.clusterTimestamp = this.content[this.content.length - 1].timestamp;
});

// assign directly to object
messageSchema.methods.objectMethod = function () {
  console.log("object method called");
};

// assign with helper
messageSchema.method("helperMethod", function () {
  console.log("helper method called");
});

console.log(messageSchema.methods); // {objectMethod: [Function (anonymous)], helperMethod: [Function (anonymous)]}
console.log(messageSchema.methodOptions); // { helperMethod: undefined }

const Message = mongoose.model("Message", messageClusterSchema);

export default Message;