Querying Documents

Learn about different query operators ($eq, $gt, $lt, $in, etc.) and how to construct complex queries to retrieve specific documents.

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MongoDB Array Operators: $all, $elemMatch, $size

Introduction

MongoDB provides powerful array operators that allow you to query and manipulate data stored in array fields. This document explores three essential array operators: $all, $elemMatch, and $size.

$all Operator

The $all operator selects documents where the specified array field contains all the elements specified in the query. The order of elements in the array does not matter.

Example

Consider a collection named products with documents like this:

 {
  "_id": ObjectId("654321abc123def456ghi789"),
  "name": "Laptop",
  "tags": ["electronics", "computer", "portable"]
}

{
  "_id": ObjectId("987654xyz321fed098uvw654"),
  "name": "Smartphone",
  "tags": ["electronics", "mobile", "camera"]
}

{
  "_id": ObjectId("123456def654abc987zyx321"),
  "name": "Book",
  "tags": ["literature", "novel", "paperback"]
}

To find all products that have both "electronics" and "computer" tags, you can use the following query:

 db.products.find({
  tags: { $all: ["electronics", "computer"] }
})

Result: This query will return the document for the "Laptop" because its `tags` array contains both "electronics" and "computer".

$elemMatch Operator

The $elemMatch operator selects documents where at least one element in the array field matches all the specified query criteria.

Example

Consider a collection named students with documents like this:

 {
  "_id": ObjectId("654321abc123def456ghi789"),
  "name": "Alice",
  "grades": [
    { "subject": "Math", "score": 90 },
    { "subject": "Science", "score": 85 }
  ]
}

{
  "_id": ObjectId("987654xyz321fed098uvw654"),
  "name": "Bob",
  "grades": [
    { "subject": "Math", "score": 70 },
    { "subject": "Science", "score": 95 }
  ]
}

To find students who have at least one grade where the subject is "Math" and the score is greater than 80, you can use the following query:

 db.students.find({
  grades: { $elemMatch: { subject: "Math", score: { $gt: 80 } } }
})

Result: This query will return the document for "Alice" because she has a grade in "Math" with a score of 90. Bob does not meet this criteria because his math score is only 70.

$size Operator

The $size operator selects documents where the specified array field is a certain size (number of elements).

Example

Using the products collection from the $all example:

 {
  "_id": ObjectId("654321abc123def456ghi789"),
  "name": "Laptop",
  "tags": ["electronics", "computer", "portable"]
}

{
  "_id": ObjectId("987654xyz321fed098uvw654"),
  "name": "Smartphone",
  "tags": ["electronics", "mobile", "camera"]
}

{
  "_id": ObjectId("123456def654abc987zyx321"),
  "name": "Book",
  "tags": ["literature", "novel", "paperback"]
}

To find all products that have exactly 3 tags, you can use the following query:

 db.products.find({
  tags: { $size: 3 }
})

Result: This query will return all three documents because each document's `tags` array has a size of 3.

To find all products that have exactly 2 tags, the result set would be empty.

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