👐 $match

The $match stage filters documents in a collection. It takes a document as input (your filter) and returns the documents that match the specified criteria. The syntax for the $match stage is as follows:

{ $match: { <expression>: <value> } }

Expressions

The <expression> portion of the $match stage can be any valid MongoDB expression. This includes:

• Comparison operators: eq, neq, gte, lte, gt, lt, in, nin, exists.
• Regular expressions: regex.
• Logical operators: and, or, not.
• Subdocuments and arrays: { field: <value> }, [ <item>, <item>, ... ].

Matching book documents

Atlas UI

First, make sure you select the books collection in the Atlas UI.

Then, navigate to the Aggregation tab and click </> TEXT.

Say we want all the books from the year 2010. We can add a $match stage to filter the documents from the books collection:

[
    {
        $match: { year: 2010 }
    }
]

MongoDB Shell

Say we want all the books from 2010. We'll write:

db.books.aggregate([
  {
    $match: {year: 2010}
  }
])

👐 Return all the books that have exactly 100 pages.

Answer

Atlas UI

[
  {
    $match: { pages: 100 }
  }
]

MongoDB Shell

db.books.aggregate([
  {
    $match: { pages: 100 }
  }
])

AND

If we need to add more conditions using AND, we use the $and operator.

If we want all the books with 100 pages and with exactly totalInventory 1, we can use the $and operator. It takes an array of documents with all the conditions that should be true for the AND to succeed:

Atlas UI

[
  {
    $match: {
      $and: [
        { pages: 100 },
        { totalInventory: 1 }
      ]
    }
  }
]

MongoDB Shell

db.books.aggregate([
  {
    $match: { 
      $and: [
        { pages: 100 }, 
        { totalInventory: 1 }
      ]
    }
  }
])

The pseudo-code for this would be something like:

IF pages == 100 AND totalInventory == 1 {
    return matching docs
}

👐 Return all the books from 2009 that have exactly 192 pages.

Answer

Atlas UI

[
  {
    $match: {
      $and: [
        { pages: 192 },
        { year: 2009 }
      ]
    }
  }
]

MongoDB Shell

db.books.aggregate([
  { 
    $match: {
      $and: [
        { pages: 192 }, 
        { year: 2009 }
      ]
    } 
  }
])

👐 How many are there? We haven't yet seen the $count stage, but try to adding a second stage to your pipeline with { $count: "books_count" }

Answer

Atlas UI

[
    {
        $match: {
            $and: [
                { pages: 100 },
                { year: 2015 }
            ]
        }
    },
    {
        $count: "books_count"
    }
]

MongoDB Shell

db.books.aggregate(
  [
    { $match: {$and: [{pages: 100}, {year: 2015}]} },
    { $count: "books_count" }
  ]
)

Shorthand AND

We can do an implicit AND just passing a document with all the conditions (instead of an array of documents):

Atlas UI

[
  {
    $match: { pages: 100, totalInventory: 2 }
  }
]

MongoDB Shell

db.books.aggregate([
  {
    $match: { pages: 100, totalInventory: 2 }
  }
])

👐 Return all the books from 2009 that have exactly 192 pages, using the shorthand $and notation:

Answer

Atlas UI

[
    {
        $match: {pages: 192, year: 2009}
    }
]

MongoDB Shell

db.books.aggregate([{$match: {pages: 192, year: 2009}}])