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πŸ‘ $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​

Open the 100_aggregation_pipeline_match file in the Instruqt IDE or GitHub Codespaces. Run the Startup Code, that will import all the neccessary modules and connect to the local MongoDB cluster. Then, run the $match cell.

Running examples in the Instruqt IDE or GitHub Codespaces.

πŸ‘ $match Exercise: Return all the books that have exactly 100 pages.​

Answer

Insert a new cell (you can do it by clicking the + Code button at the top left of the notebook) and write the following code:

const booksWith100pages = [
    { $match: { 
        pages: 100
    } 
    }
 ];
 
 const cursor = await books.aggregate( booksWith100pages );
 
 await cursor.forEach((b) => {
    console.log(b);
 });

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:

Insert a new cell (you can do it by clicking the + Code button at the top left of the notebook) and write the following code:

const booksWith100pagesAndTotalInventory1 = [
    { $match: { 
        pages: 100,
        totalInventory: 1
    } 
    }
 ];

 const cursor = await books.aggregate( booksWith100pagesAndTotalInventory1 );

 await cursor.forEach((b) => {
    console.log(b);
 });

The pseudo-code for this would be something like:

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

πŸ‘ Return all the books from 2015 that have exactly 100 pages.

Answer

Insert a new cell (you can do it by clicking the + Code button at the top left of the notebook) and write the following code:

const booksWith100pagesFrom2015 = [
  {
    $match: {
      $and: [
        { pages: 100 },
        { year: 2015 }
      ]
    }
  }
 ];

 const cursor = await books.aggregate( booksWith100pagesFrom2015 );

 await cursor.forEach((b) => {
    console.log(b);
 });

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

Answer

Insert a new cell (you can do it by clicking the + Code button at the top left of the notebook) and write the following code:

const countBooksWith100pagesFrom2015 = [
  {
  $match: {
    $and: [
      { pages: 100 },
      { year: 2015 }
    ]
  }
  },
  {
    $count: "books_count"
  }
 ];

 const cursor = await books.aggregate( countBooksWith100pagesFrom2015 );

 await cursor.forEach((b) => {
    console.log(b);
 });

Shorthand AND​

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

Insert a new cell (you can do it by clicking the + Code button at the top left of the notebook) and write the following code:

const booksWith100pagesWithInventory2 = [
  {
  $match: { 
    pages: 100, 
    totalInventory: 2 }
  }
 ];

 const cursor = await books.aggregate( booksWith100pagesWithInventory2 );

 await cursor.forEach((b) => {
    console.log(b);
 });

πŸ‘ Return all the books from 2015 that have exactly 100 pages, using the shorthand $and notation:

Answer

Insert a new cell (you can do it by clicking the + Code button at the top left of the notebook) and write the following code:

const booksWith100pagesFrom2015 = [
  {
  $match: { 
    pages: 100, 
    year: 2015 
  }
  }
 ];

 const cursor = await books.aggregate( booksWith100pagesFrom2015 );

 await cursor.forEach((b) => {
    console.log(b);
 });