如何用Mongoose 4.13.11查询数组元素字段不匹配的文档？

Hey there! Let's work through this query challenge you're dealing with. Since you're on Mongoose 4.13.11, we need a solution that's compatible with that version, and both $expr and $where can work here—let's break down the best approaches.

Option 1: Use $expr (Recommended for Better Performance) #

MongoDB 3.6+ (which Mongoose 4.13.x supports) introduced $expr, letting you compare fields within the same document directly in query filters. To check if any element in your array has a last_message_read.messageId that doesn't match the document's last_message, we can combine $expr with $map and $anyElementTrue:

// Replace "YourModel" with your actual model name, and adjust field names as needed
YourModel.find({
  user_id: "target-user-id", // Add your user_id filter here
  $expr: {
    $anyElementTrue: {
      $map: {
        input: "$yourArrayField", // Replace with your array field's name
        as: "arrayItem",
        in: { $ne: ["$$arrayItem.last_message_read.messageId", "$last_message"] }
      }
    }
  }
})
  .then(docs => console.log("Matching documents:", docs))
  .catch(err => console.error("Query error:", err));

How this works: #

• $map iterates over every element in your array, returning a boolean array where each value is true if the element's last_message_read.messageId doesn't equal the document's last_message.
• $anyElementTrue checks if there's at least one true in that boolean array—if yes, the document matches the filter.
• We add your user_id condition directly in the query object, so it combines seamlessly with the $expr logic.

Option 2: Use $where (Alternative for Edge Cases) #

If for some reason $expr isn't working for you, $where lets you write a JavaScript function to evaluate the condition. Note that this is less performant than $expr (since it runs JS on each document), but it's a valid fallback:

YourModel.find({
  user_id: "target-user-id",
  $where: function() {
    // Add checks to avoid errors if nested fields are missing
    if (!this.yourArrayField || !this.last_message) return false;
    
    return this.yourArrayField.some(arrayItem => {
      return arrayItem.last_message_read && 
             arrayItem.last_message_read.messageId !== this.last_message;
    });
  }
})

Important notes for $where: #

• Always add checks for nested fields (like arrayItem.last_message_read) to avoid runtime errors if some documents don't have those fields.
• Avoid using user input directly in $where functions—this can lead to security vulnerabilities.

Both approaches let you combine the user_id condition with your array mismatch check. Stick with $expr whenever possible for better query performance and compatibility with MongoDB's query optimizer.

内容的提问来源于stack exchange，提问作者Landon Vago-Hughes