我正在为项目创建express服务器，并使用mongodb作为数据库。我得到了错误，

Error validating $lookup value. err=$lookup against an expression value is not allowed in this atlas tier.

电影模式，
const mongoose = require（“mongoose”）;

const MovieSchema = new mongoose.Schema(
  {
    title: { type: String, required: true, unique: true },
    desc: { type: String },
    img: { type: String },
    imgTitle: { type: String },
    imgSm: { type: String },
    trailer: { type: String },
    video: { type: String },
    year: { type: String },
    limit: { type: Number },
    genre: { type: String },
    isSeries: { type: Boolean, default: false },
  },
  { timestamps: true }
);

module.exports = mongoose.model("movies", MovieSchema);

我的列表架构，

const ListSchema = new mongoose.Schema(
  {
    title: { type: String, required: true, unique: true },
    type: { type: String },
    genre: { type: String },
    content: [
      { type: mongoose.Schema.Types.ObjectId, required: false, ref: "movies" },
    ],
  },
  { timestamps: true }
);

lists schema有一个content属性，它包含movies的id。所以我需要在list中获取所有movies，sample大小为4，所以我在list schema的content属性中写入aggregation作为列表模式，用于填充movies对象。我的代码中的bug在哪里？希望对您有所帮助！所以我把它编码为

const list = await List.aggregate([
            { $sample: { size: 4 } },
            {
              $lookup: {
                from: "$movies",
                foreignField: "_id",
                localField: "content",
                as: "content",
              },
            },
          ]);
        }
        res.status(200).json(list);