Choose the initial element from the nested array

Question

Choose the initial element from the nested array

Is there a way to retrieve the first item from a nested Array without having to fetch the entire document?

Data Schema/Model

Consider the following Schema:

const parentSchema = mongoose.Schema({
  name: String,
  children: []
});

const grandparentSchema = mongoose.Schema({
  name: String,
  children: [parentSchema]
})

This schema translates into the example instance shown below:

{
  name: 'Grandparent Foo',
  children: [
    {
      name: 'Parent Foo',
      children: ['Child Foo', 'Child Bar', 'Child Baz']
    }
  ]
}

The Query

I am trying to access the first child of 'Parent Foo'. Essentially, I want to retrieve 'Child Foo'.

Important Details

In this scenario, the grandchildren are stored as plain strings and not as separate documents (unlike the Parent), making it impossible to select them using dot notation.
I do not wish to fetch the entire document and filter through it in my code. My goal is to only transmit the first grandchild over the network as the children array of 'Parent Foo' might contain an immense number of entries.
The reason behind this query is that I intend to use $pop method on the first grandchild after fetching it. Therefore, I need to fetch the item first and then perform the $pop operation on it.

javascript mongodb mongoose mongodb-query aggregation-framework

Answer 1

Answer №1

In order to retrieve only specific data from the database, additional work may be required.

To explain in general terms:

Grandparent.find(
  { "children.name": "Parent Foo" },
  { "children.$": 1 }
)

This code snippet will fetch the matched entry from "children" without returning any other entries if they exist.

If you specifically need the first element of an array, then you can use .aggregate() like this:

Granparent.aggregate([
  { "$match": { "children.name": "Parent Foo" } },
  { "$addFields": {
    "children": {
      "$map": {
        "input": {
          "$filter": {
            "input": "$children",
            "as": "child",
            "cond": { "$eq": [ "$$child.name", "Parent Foo" ] }
          }
        },
        "as": "child",
        "in": {
          "name": "$$child.name",
          "children": { "$arrayElemAt": [ "$$child.children", 0 ] }
        }
      }
    }
  }}
])

This approach involves using $filter and $map to manipulate the data as needed.

If you can tolerate a bit of extra data being returned, you can simply slice off the positional match:

Grandparent.find(
  { "children.name": "Parent Foo" },
  { "children.$": 1 }
).lean().exec((err,docs) => {
  docs = docs.map( doc => {
    doc.children = doc.children.map( c => c.children = c.children.slice(0,1) );
    return doc;
  });
  // do something with docs

In this case, a small amount of additional data is returned but can easily be trimmed on the client side with minimal effort.

The effectiveness of this method may vary depending on the size of the actual data, but usually, trimming data on the client side is preferable when the difference is negligible.

Answer 2

In order to retrieve only specific data from the database, additional work may be required.

To explain in general terms:

Grandparent.find(
  { "children.name": "Parent Foo" },
  { "children.$": 1 }
)

This code snippet will fetch the matched entry from "children" without returning any other entries if they exist.

If you specifically need the first element of an array, then you can use .aggregate() like this:

Granparent.aggregate([
  { "$match": { "children.name": "Parent Foo" } },
  { "$addFields": {
    "children": {
      "$map": {
        "input": {
          "$filter": {
            "input": "$children",
            "as": "child",
            "cond": { "$eq": [ "$$child.name", "Parent Foo" ] }
          }
        },
        "as": "child",
        "in": {
          "name": "$$child.name",
          "children": { "$arrayElemAt": [ "$$child.children", 0 ] }
        }
      }
    }
  }}
])

This approach involves using $filter and $map to manipulate the data as needed.

If you can tolerate a bit of extra data being returned, you can simply slice off the positional match:

Grandparent.find(
  { "children.name": "Parent Foo" },
  { "children.$": 1 }
).lean().exec((err,docs) => {
  docs = docs.map( doc => {
    doc.children = doc.children.map( c => c.children = c.children.slice(0,1) );
    return doc;
  });
  // do something with docs

In this case, a small amount of additional data is returned but can easily be trimmed on the client side with minimal effort.

The effectiveness of this method may vary depending on the size of the actual data, but usually, trimming data on the client side is preferable when the difference is negligible.

Choose the initial element from the nested array

Data Schema/Model

The Query

Important Details

Answer №1

Similar questions

"Unleashing the power of custom servers to tap into the rendered HTML of Next

Error: The function mongoclient.db cannot be executed

Tips for finding the displayRows paragraph within the MUI table pagination, nestled between the preceding and succeeding page buttons

Exploring Substrings in jQuery/JavaScript

Angular: Design dependent on attributes

Leverage AJAX to fetch data from the database

Error in Mathquill: Unable to access the 'prototype' property because it is undefined

Is there a way to create tabs in JavaScript without having to edit the <head> section?

Nextjs unexpectedly displays blank page upon fetching data from Firebase Firestore without any error messages

How can I convert a string containing integers into an int[] using javascript or jQuery?

"Troubleshooting when a NodeJS Program Refuses to

Building an accordion collapse feature in HTML using CSS and JavaScript

Creating an Active Link in Bootstrap 5.0 (Beta 3) Navbar Using JavaScript

When iterating through a JavaScript object, opt for utilizing references instead of repeatedly specifying the path

Managing the status (e.g. 503) for Axios OPTIONS response

Best method for simultaneously calling multiple routes in Node.js and Express

Enhanced jQuery implementation for hiding elements

Change the HTML table's toggle element to either 'on' or 'off' based on the specified value using jQuery

Mapping drop-downs based on the length of a JSON array

Encountering a crash issue with JMeter's Selenium Sampler while attempting to click on a button with the Phantom