title: mongoose how do I call other model in other schema definition or update schema in route?
body: <p>I have a model like this:</p>
  <pre><code>const mongoose = require('mongoose');
  const schemaOpts = { toJSON: { virtuals: true } };
  const { Schema } = mongoose;
  const { v4: uuidv4 } = require('uuid');
  const IdentitySchema = require('./Identity');
  const Title = require('./Title');
  
  var titleSchema = Title.schema.tree.series[0];
  
  const a = async function() {
      return await Title.findOne({_id: 'titles'})
  };
  
  titleSchema.a.enum = a();
  
  const schema = new Schema({
      short_id: {
          type: String,
          default: uuidv4().slice(0, 8),
      },
      name: {
          type: String,
          required: [true, 'required']
      },
      FIELD: {
          type: String,
          required: [true, 'required']
      },
  })
  
  const UserModel = mongoose.model('users', schema)
  
  module.exports = UserModel
  </code></pre>
  <p>How do I get a() function value? Or in this place I can only use fixed code? I think other way to do this is modify it in route but I google it can not. I'm using expressjs 4.16.1 and mongoose 6.3.4.</p>