$lookup with pipeline (advanced join) in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the $lookup stage in MongoDB aggregation?

The $lookup stage is used to perform a left outer join to another collection in the same database, allowing you to combine documents from two collections based on a related field.

intermediate

How does $lookup with a pipeline differ from a simple $lookup with localField and foreignField?

Using $lookup with a pipeline allows you to perform more complex operations like filtering, projecting, and sorting on the joined collection before joining, unlike the simple field-to-field equality match.

intermediate

In a $lookup with pipeline, what does the let field do?

The let field defines variables from the current collection's document that can be used inside the pipeline stages to reference values dynamically during the join.

advanced

Write a simple example of a $lookup stage using pipeline to join orders with products where the product price is greater than 100.

Example:

{
  $lookup: {
    from: "products",
    let: { product_id: "$productId" },
    pipeline: [
      { $match: { $expr: { $and: [ { $eq: ["$_id", "$$product_id"] }, { $gt: ["$price", 100] } ] } } }
    ],
    as: "expensiveProducts"
  }
}

beginner

What is the output of a $lookup stage with pipeline if no matching documents are found in the joined collection?

The output will include the original document with the joined field as an empty array, indicating no matches were found.

What does the as field specify in a $lookup stage?

A The name of the new array field to add the joined documents

B The collection to join with

C The local field to match

D The foreign field to match

The as field defines the name of the array field in the output documents where the joined documents will be stored.

Which operator allows you to use variables from the current document inside the $lookup pipeline?

A $let

B let

C $expr

D $match

The let field defines variables from the current document that can be referenced inside the $lookup pipeline.

In $lookup with pipeline, how do you compare fields from the joined collection and the outer collection?

A Using <code>$eq</code> inside <code>$expr</code> with <code>$$</code> to reference variables

B Using <code>$match</code> with field names directly

C Using <code>$group</code> stage

D Using <code>$project</code> stage

You use $expr with comparison operators like $eq and reference outer variables with $$ inside the $lookup pipeline.

What type of join does $lookup perform in MongoDB?

A Inner join

B Full outer join

C Right outer join

D Left outer join

$lookup performs a left outer join, meaning all documents from the left collection are included, with matching documents from the right collection if available.

If you want to filter joined documents in $lookup, which feature should you use?

A localField and foreignField

B as field

C pipeline with $match

D sort stage

Using pipeline with a $match stage inside $lookup lets you filter the joined documents before adding them.

Explain how $lookup with pipeline works and why it is useful compared to a simple $lookup.

Think about how you can filter and transform joined data before adding it.

Describe the role of the let and pipeline fields inside a $lookup stage.

Consider how variables help connect fields from two collections.