如何转换authorid关联postid提取作者信息存入authorDocument数组及代码排查

• 语法错误：代码中的"是HTML转义后的双引号符号，不属于JavaScript合法语法，直接运行会抛出语法错误，需要先替换为标准的双引号"。
• 关联字段类型不匹配：MongoDB的$lookup要求关联的两个字段类型完全一致才能匹配成功。你的代码已经通过$addFields将字符串格式的作者ID转换为ObjectId类型存储在authorId字段，但后续$lookup的localField仍使用了原字符串类型的author字段，和users集合的_id（默认是ObjectId类型）类型不匹配，导致关联失败拿不到作者数据。

let posts = await postsCollection.aggregate([
  {$match: {_id: new ObjectID(id)}},
  {$addFields: {authorId: { $toObjectId: "$author"}}},
  // 替换localField为转换后的ObjectId类型字段即可
  {$lookup: {from: "users", localField: "authorId", foreignField: "_id", as: "authorDocument"}}
]).toArray()

如果你的posts集合中author字段本身已经是ObjectId类型，可以直接省略$addFields阶段简化代码：

let posts = await postsCollection.aggregate([
  {$match: {_id: new ObjectID(id)}},
  {$lookup: {from: "users", localField: "author", foreignField: "_id", as: "authorDocument"}}
]).toArray()

如果单篇帖子只有一个作者，还可以在聚合最后增加一步把返回的数组类型authorDocument转成单个对象，更方便后续调用：

let posts = await postsCollection.aggregate([
  {$match: {_id: new ObjectID(id)}},
  {$addFields: {authorId: { $toObjectId: "$author"}}},
  {$lookup: {from: "users", localField: "authorId", foreignField: "_id", as: "authorDocument"}},
  {$addFields: {authorDocument: {$first: "$authorDocument"}}}
]).toArray()

内容的提问来源于stack exchange，提问作者Bivishan Sapkota