Mongoose中$lookup结合$unwind使用$size统计报错求助

Hey there, let's figure out why you're hitting that MongoError and fix your aggregation query properly.

What's Causing the Error? #

Your mistake comes from two key issues with your aggregation stages:

1. $unwind turns your array into a single object
   After running $unwind: "$replacement_articles", the replacement_articles field is no longer an array—it's a single document from the originally joined array. When you try to use $size on it, MongoDB throws an error because $size requires an array, not a single object (or missing value if there were no matches).
2. Incorrect stage and operator for counting
   Your final stage is structured like a projection but uses $push, which is for collecting values into an array—not counting. To tally the number of related articles, you need either a $project with $size (if you skip $unwind) or a $group with $sum.

Fix 1: Skip $unwind (Most Efficient) #

Since $lookup already returns an array of related replacement_articles, you can directly use $size to get the count without splitting the array. This is way more efficient because it avoids creating extra documents with $unwind.

aggregate([
  {
    $lookup: {
      from: "replacement_articles",
      localField: "_id",
      foreignField: "userid",
      as: "replacement_articles"
    }
  },
  {
    $project: {
      _id: "$_id", // Keep the original document ID
      article_count: { $size: "$replacement_articles" } // Get length of the joined array
    }
  }
])

Fix 2: Use $unwind + $group (If You Need Per-Article Processing) #

If you must use $unwind (e.g., to run additional operations on individual replacement articles), you'll need to group back by the original document ID and count with $sum: 1. Also, add preserveNullAndEmptyArrays: true to keep users who have no related articles (otherwise they'll be filtered out).

aggregate([
  {
    $lookup: {
      from: "replacement_articles",
      localField: "_id",
      foreignField: "userid",
      as: "replacement_articles"
    }
  },
  {
    $unwind: {
      path: "$replacement_articles",
      preserveNullAndEmptyArrays: true // Retain users with no related articles
    }
  },
  {
    $group: {
      _id: "$_id", // Group by the original user document's ID
      article_count: { $sum: 1 } // Count how many documents are in each group
    }
  }
])

内容的提问来源于stack exchange，提问作者Naresh P