This is the third day of my participation in the November Gwen Challenge. Check out the details: the last Gwen Challenge 2021

Mongoose’s Array class extends normal JavaScript arrays with additional Mongoose functionality.

For example, suppose you have a BlogPost model skeleton with an array of tags:

const blogPostSchema = Schema({
  title: String.tags: [String]})Copy the code

When you create a new BlogPost document, the Tags attribute is an instance of a normal JavaScript array class. But it also has some special properties.

const blogPostSchema = Schema({
  title: String.tags: [String] {},versionKey: false })
const BlogPost = mongoose.model('BlogPost', blogPostSchema)
​
const doc = new BlogPost({
  title: 'Intro to JavaScript'.tags: ['programming']})Array.isArray(doc.tags) // true
doc.tags.isMongooseArray // true
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For example, Mongoose intercepts push() calls to the tags array and is smart enough to update the document with $push when you save().

mongoose.set('debug'.true)
​
doc.tags.push('web development')
// Due to debug mode, print:
// Mongoose: blogposts.updateOne({ _id: ObjectId(...) }, { '$push': { tags: { '$each': [ 'web development' ] } } }, { session: null })
await doc.save()
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Document the array

The tags example is a primitive array. Mongoose also supports subdocument arrays. Here’s how to define the Members array, each with firstName and lastName attributes.

const groupSchema = Schema({
  name: String.members: [{ firstName: String.lastName: String}]})Copy the code

Doc.members is an instance of a normal JavaScript array, so it has all the usual functions, such as slice() and filter(). But it also has some functions specific to Mongoose.

const groupSchema = Schema({
  name: String.members: [{ firstName: String.lastName: String}]})const Group = mongoose.model('Group', groupSchema)
​
const doc = new Group({
  title: 'Jedi Order'.members: [{ firstName: 'Luke'.lastName: 'Skywalker'}]})Array.isArray(doc.members) // true
doc.members.isMongooseArray // true
doc.members.isMongooseDocumentArray // true
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For example, if you set firstName for the 0th member, Mongoose will convert it to a collection on member.0.firstName when save() is called.

const groupSchema = Schema({
  name: String.members: [{ firstName: String.lastName: String}]}, {versionKey: false })
const Group = mongoose.model('Group', groupSchema)
​
const doc = new Group({
  title: 'Jedi Order'.members: [{ firstName: 'Luke'.lastName: 'Skywalker'}]})await doc.save()
​
mongoose.set('debug'.true)
​
doc.members[0].firstName = 'Anakin'
// Mongoose: groups.updateOne({ _id: ObjectId("..." ) }, { '$set': { 'members.0.firstName': 'Anakin' } }, { session: null })
await doc.save()
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Considerations when setting an array index

Mongoose has a known problem with setting array indexes directly. For example, if you set doc.tags[0], the Mongoose change trace will not get the change.

const blogPostSchema = Schema({
  title: String.tags: [String] {},versionKey: false })
const BlogPost = mongoose.model('BlogPost', blogPostSchema)
​
const doc = new BlogPost({
  title: 'Intro to JavaScript'.tags: ['programming']})await doc.save()
​
// This change does not end up in the database!
doc.tags[0] = 'JavaScript'
await doc.save()
​
const fromDb = await BlogPost.findOne({ _id: doc._id })
console.log(fromDb.tags) // ['programming']
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To solve this problem, you need to use the markModified() method or explicitly call MongoosearRay.set () on an array element to notify Mongoose of the change trace, as shown below.

// The change worked. Set () is a special way to trigger change tracing on a Mongoose array.
doc.tags.set(0.'JavaScript')
await doc.save()
​
const fromDb = await BlogPost.findOne({ _id: doc._id })
console.log(fromDb.tags) // ['JavaScript']
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