MongoDB数组统计与条件切片问题：现有查询返回null求解

Let's break down why your query is returning null and how to fix it step by step:

Key Issues in Your Original Query #

1. Lost list Field in Projection: Your first $project stage only keeps the notifs field, which drops the original list array from the pipeline. When the second $project tries to reference $list, it no longer exists—this is the main reason you're getting null.
2. Invalid ObjectId: The profileId in your $match stage has an invalid character (s instead of a hexadecimal digit). ObjectIds require exactly 24 hex characters (0-9, a-f).
3. Redundant Slice for Full Array: Using $slice: ["$list", "$notifs"] to get all elements works, but it's simpler to just return the array directly.

Corrected Query #

db.getCollection('post').aggregate([
  // Match the valid profileId from your sample document
  { $match: { profileId: ObjectId("5ad84c8c0e71892058b6a543") } },
  {
    $project: {
      notifications: {
        $cond: [
          // Check if the list has 10 or more elements
          { $gte: [{ $size: "$list" }, 10] },
          // Return first 10 elements if true
          { $slice: ["$list", 10] },
          // Return the entire list if false
          "$list"
        ]
      }
    }
  }
])

Explanation #

• Single $project Stage: We calculate the size of list inline using $size within the $cond check, so we never lose access to the original list array.
• Valid ObjectId: We fixed the typo in the profileId to match the one from your sample document.
• Simplified Logic: When the list is shorter than 10 elements, we directly return the full array instead of using $slice, making the query cleaner and more efficient.

Alternative (Include Total Notification Count) #

If you also want to return the total number of notifications alongside the sliced list, you can add it to the projection:

db.getCollection('post').aggregate([
  { $match: { profileId: ObjectId("5ad84c8c0e71892058b6a543") } },
  {
    $project: {
      totalNotifications: { $size: "$list" },
      notifications: {
        $cond: [
          { $gte: [{ $size: "$list" }, 10] },
          { $slice: ["$list", 10] },
          "$list"
        ]
      }
    }
  }
])

This will give you both the total count of notifications and the appropriate sliced/full array.

内容的提问来源于stack exchange，提问作者Raju Adhikari