MongoError：WiredTigerIndex :: insert：键太大而无法索引，失败23012，（尝试保存大字符串时失败）

Question

我正在尝试将JSON字符串保存到mongoDB，似乎字符串的长度太大，并且出现以下错误消息。

MongoError: WiredTigerIndex::insert: key too large to index, failing  23012

我的架构非常简单，

const metadataSchema = new Schema({
  userId: { type: String, unique: false },
  metadataJSON: { type: String, unique: false, failIndexKeyTooLong: false }
});

const saveMetadata = async (userId, customMetadata) => {
  let metadata = METADATA({
    userId: userId,
    metadataJSON: JSON.stringify(customMetadata)
  });
  metadata.save(err => {
    if (err)
      console.log(
        "ERROR - Metadata was not inserted and here is the reason : " + err
      );
    else console.log("All is good, metadata was inserted");
  });
};

关于如何插入此大字符串的任何建议？预先感谢。