Updating nested array subdocuments based on a field in the main document is a common task when working with MongoDB and Mongoose. Here’s a simple guide and example to help you accomplish this.

Plan

  1. Use Model.updateMany() or Model.findOneAndUpdate() to update multiple or a single document.
  2. Use an aggregation pipeline ([{}]) as the second argument to reference document fields.
  3. Use the $set stage within the pipeline to update the nested array.
  4. Use the $$ROOT variable to reference fields from the main document.

Example

Assume you have a User model where each user has an array of posts, and you want to set each post’s authorName field based on the user’s name field.

Here’s how you can do it:

const mongoose = require('mongoose');
const { Schema } = mongoose;

const postSchema = new Schema({
  title: String,
  content: String,
  authorName: String, // This is what we want to update
});

const userSchema = new Schema({
  name: String,
  posts: [postSchema],
});

const User = mongoose.model('User', userSchema);

async function updatePostAuthorNames() {
  await User.updateMany(
    {}, // Update condition, {} means all documents
    [
      {
        $set: {
          posts: {
            $map: {
              input: "$posts",
              as: "post",
              in: {
                $mergeObjects: [
                  "$$post",
                  { authorName: "$name" } // Set authorName in each post to the user's name
                ]
              }
            }
          }
        }
      }
    ]
  );
}

updatePostAuthorNames().then(() => console.log('Update complete.'));

Explanation

  1. Define Schemas: Define the postSchema and userSchema.
  2. Update Logic:
    • Use updateMany() to update all user documents.
    • Use an aggregation pipeline to modify the documents.
    • The $set stage updates the posts array.
    • The $map operator iterates over the posts array.
    • The $mergeObjects operator merges the original post object ($$post) with a new object containing the updated authorName set to the user’s name.

Running the Update

To run the update function, simply call it:

updatePostAuthorNames().then(() => console.log('Update complete.'));

This script updates the authorName for all posts based on their respective user’s name.

Conclusion

Updating nested array subdocuments using a main document field value in MongoDB with Mongoose is straightforward with the right approach. Using aggregation pipelines and Mongoose’s powerful querying capabilities, you can efficiently perform these updates.