Querying Embedded Fields and Arrays | CodeSignal Learn

Querying Embedded Fields and Arrays | CodeSignal Learn

Introduction

Welcome! In this lesson, we’ll explore how to query nested fields and arrays in MongoDB. Additionally, we will cover two comparison operators, $in and $nin, that were not discussed in the previous lesson. Let's get started!

Querying Nested Fields

When documents contain nested fields, like sub-documents, querying can be achieved using dot notation. For example, consider a document in our comic_book_store_db database: JSON{ "title": "The Amazing Spider-Man", "issue_number": 1, "published_year": 1963, "writer": { "name": "Stan Lee", "nationality": "American" } // Other fields } Let's say we want to find a comic book where the writer's name is "Stan Lee". Here's how you do it: JavaScriptuse comic_book_store_db db.comic_books.findOne( { "writer.name": "Stan Lee" }, { title: 1, writer: 1, _id: 0 } ) This query searches within the sub-document writer for the field name. Note that writer.name should be in quotes; otherwise, the . will cause an error in your query.

Querying Arrays

Arrays are common in many documents. Our comic_book_store_db has fields like genres and characters, which are arrays. Here's an example of a document containing arrays: JSON{ "title": "The Amazing Spider-Man", "issue_number": 1, "published_year": 1963, "genres": ["Action", "Adventure", "Superhero"] "characters": [ { "name": "Spider-Man", "real_name": "Peter Parker" }, { "name": "Green Goblin", "real_name": "Norman Osborn" } ] // Other fields } Here are some examples of querying arrays in MongoDB: JavaScriptuse comic_book_store_db // Locate a comic book where the genre includes "Adventure" db.comic_books.findOne( { genres: "Adventure" }, // Note that we're not using brackets here { title: 1, genres: 1, _id: 0 } ) // Find a comic book which has an exact array of genres, in the specified order db.comic_books.findOne( { genres: ["Action", "Adventure", "Superhero"] }, // Note the brackets! { title: 1, genres: 1, _id: 0 } ) // Find a comic book where a character named "Spider-Man" appears db.comic_books.findOne( { "characters.name": "Spider-Man" }, { title: 1, "characters.name": 1, _id: 0 } )

`$in` in Action

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The $in operator is used to match any value within an array of specific values. If we want a comic book that belongs to either 'Action' or 'Adventure' genres: use comic_book_store_db db.comic_books.findOne( { genres: { $in: ["Action", "Adventure"] } }, { title: 1, genres: 1, _id: 0 } )

Using `$nin`

Conversely, the $nin operator matches any value that does not exist in the specified array. To find a comic book where the writer's nationality is not "American" or "British": use comic_book_store_db db.comic_books.findOne( { "writer.nationality": { $nin: ["American", "British"] } }, { title: 1, writer: 1, _id: 0 } )