Using MongoDB with Mongoose — Query Limits and Child Refs

To make MongoDB database manipulation easy, we can use the Mongoose NPM package to make working with MongoDB databases easier.

In this article, we’ll look at how to use Mongoose to manipulate our MongoDB database.

limit vs. perDocumentLimit

Populate has a limit option, but it doesn’t support limit on a per-document basis.

For example, we can write:

async function run() {
  const { createConnection, Types, Schema } = require('mongoose');
  const connection = createConnection('mongodb://localhost:27017/test');
  const personSchema = Schema({
    _id: Schema.Types.ObjectId,
    name: String,
    age: Number,
    stories: [{ type: Schema.Types.ObjectId, ref: 'Story' }]
  });
  const storySchema = Schema({
    author: { type: Schema.Types.ObjectId, ref: 'Person' },
    title: String,
    fans: [{ type: Schema.Types.ObjectId, ref: 'Person' }]
  });
  const Story = connection.model('Story', storySchema);
  const Person = connection.model('Person', personSchema);
  const author = new Person({
    _id: new Types.ObjectId(),
    name: 'James Smith',
    age: 50
  });
  await author.save();
  const fan = new Person({
    _id: new Types.ObjectId(),
    name: 'Fan Smith',
    age: 50
  });
  await fan.save();
  const story1 = new Story({
    title: 'Mongoose Story',
    author: author._id,
    fans: [fan._id]
  });
  await story1.save();
  const story = await Story.findOne({ title: 'Mongoose Story' })
    .populate({
      path: 'fans',
      options: { limit: 2 }
    })
    .exec();
  console.log(story.fans)
}
run();

to query up to numDocuments * limit .

Mongoose 5.9.0 or later supports the perDocumentLimit property to add a per-document limit.

For example, we can write:

async function run() {
  const { createConnection, Types, Schema } = require('mongoose');
  const connection = createConnection('mongodb://localhost:27017/test');
  const personSchema = Schema({
    _id: Schema.Types.ObjectId,
    name: String,
    age: Number,
    stories: [{ type: Schema.Types.ObjectId, ref: 'Story' }]
  });
  const storySchema = Schema({
    author: { type: Schema.Types.ObjectId, ref: 'Person' },
    title: String,
    fans: [{ type: Schema.Types.ObjectId, ref: 'Person' }]
  });
  const Story = connection.model('Story', storySchema);
  const Person = connection.model('Person', personSchema);
  const author = new Person({
    _id: new Types.ObjectId(),
    name: 'James Smith',
    age: 50
  });
  await author.save();
  const fan = new Person({
    _id: new Types.ObjectId(),
    name: 'Fan Smith',
    age: 50
  });
  await fan.save();
  const story1 = new Story({
    title: 'Mongoose Story',
    author: author._id,
    fans: [fan._id]
  });
  await story1.save();
  const story = await Story.findOne({ title: 'Mongoose Story' })
    .populate({
      path: 'fans',
      perDocumentLimit: 2
    })
    .exec();
  console.log(story.fans)
}
run();

Refs to Children

If we call push to items to children, then we can get the refs to the child items.

For example, we can write:

async function run() {
  const { createConnection, Types, Schema } = require('mongoose');
  const connection = createConnection('mongodb://localhost:27017/test');
  const personSchema = Schema({
    _id: Schema.Types.ObjectId,
    name: String,
    age: Number,
    stories: [{ type: Schema.Types.ObjectId, ref: 'Story' }]
  });
  const storySchema = Schema({
    author: { type: Schema.Types.ObjectId, ref: 'Person' },
    title: String,
    fans: [{ type: Schema.Types.ObjectId, ref: 'Person' }]
  });
  const Story = connection.model('Story', storySchema);
  const Person = connection.model('Person', personSchema);
  const author = new Person({
    _id: new Types.ObjectId(),
    name: 'James Smith',
    age: 50
  });
  await author.save();
  const fan = new Person({
    _id: new Types.ObjectId(),
    name: 'Fan Smith',
    age: 50
  });
  await fan.save();
  const story1 = new Story({
    title: 'Mongoose Story',
    author: author._id,
  });
  story1.fans.push(fan);
  await story1.save();
  const story = await Story.findOne({ title: 'Mongoose Story' })
    .populate('fans')
    .exec();
  console.log(story.fans)
}
run();

We call push on story.fans to add an entry to the fans array field.

Now when we query the story, we get the fans array with the fan in it.

Conclusion

We can limit how many documents are returned and add entries to array fields so that we can access the refs to child entries.