🦸‍♂️ Repeating Stages

info

Extra activity! Do it if you have extra time or are following along at home. It won't be covered during the hands-on lab.

From the aggregation pipelines manual:

All stages except the $out, $merge, $geoNear, $changeStream, and $changeStreamSplitLargeEvent stages can appear multiple times in a pipeline.

So we can repeat most stages, and do something like this to get all books from 1985 with more than 100 pages (although it may or may not make sense):

Atlas UI

[
  { $match: { pages: {$gte: 100} } },
  { $match: { year: 1985 } }
]

MongoDB Shell

db.books.aggregate([
  { $match: { pages: {$gte: 100} } },
  { $match: { year: 1985 } }
])

💻 Add several $limit stages at the end of the above aggregation, limiting to 1 book, and see what happens.

Answer

Atlas UI

[
    {$match: {pages: {$gte: 100}}}, 
    {$match: {year: 2011}},
    {$limit: 1},
    {$limit: 1},
]

MongoDB Shell

db.books.aggregate([
    {$match: {pages: {$gte: 100}}}, 
    {$match: {year: 2011}},
    {$limit: 1},
    {$limit: 1},
])