facet

Definition

The facet stage processes multiple aggregation pipelines within a single stage on the same set of input documents. Each sub-pipeline has its own field in the output document where its results are stored as an array of documents.

Read the official MongoDB documentation

Syntax

facet has the following form:

facet 
    name (pipelineitem)* 
    (, name (pipelineitem)*)*
end

Behaviour

facet can use any pipeline stages except:

• collStats
• facet
• geoNear
• indexStats
• out
• merge
• planCacheStats

Each sub-pipeline within facet is passed the exact same set of input documents. These sub-pipelines are completely independent of one another and the document array output by each is stored in separate fields in the output document. The output of one sub-pipeline can not be used as the input for a different sub-pipeline within the same facet stage. If further aggregations are required, add additional stages after facet and specify the field name of the desired sub-pipeline output.

Example

The following example takes a json array of objects and feeds them into an aggregation pipeline using the documents pipeline stage. It then uses MongoDB's faceting features to provide customers with the store's inventory categorized across multiple dimensions such as tags, price, and year created. This facet stage has three sub-pipelines that use

• sortByCount
• bucket
• bucketAuto

to perform this multi-faceted aggregation.

json = `[{ "_id" : 1, "title" : "The Pillars of Society", "artist" : "Grosz", "year" : 1926,
  "price" : 199.99,
  "tags" : [ "painting", "satire", "Expressionism", "caricature" ] },
{ "_id" : 2, "title" : "Melancholy III", "artist" : "Munch", "year" : 1902,
  "price" : 280.00,
  "tags" : [ "woodcut", "Expressionism" ] },
{ "_id" : 3, "title" : "Dancer", "artist" : "Miro", "year" : 1925,
  "price" : 76.04,
  "tags" : [ "oil", "Surrealism", "painting" ] },
{ "_id" : 4, "title" : "The Great Wave off Kanagawa", "artist" : "Hokusai",
  "price" : 167.30,
  "tags" : [ "woodblock", "ukiyo-e" ] },
{ "_id" : 5, "title" : "The Persistence of Memory", "artist" : "Dali", "year" : 1931,
  "price" : 483.00,
  "tags" : [ "Surrealism", "painting", "oil" ] },
{ "_id" : 6, "title" : "Composition VII", "artist" : "Kandinsky", "year" : 1913,
  "price" : 385.00,
  "tags" : [ "oil", "painting", "abstract" ] },
{ "_id" : 7, "title" : "The Scream", "artist" : "Munch", "year" : 1893,
  "tags" : [ "Expressionism", "painting", "oil" ] },
{ "_id" : 8, "title" : "Blue Flower", "artist" : "O'Keefe", "year" : 1918,
  "price" : 118.42,
  "tags" : [ "abstract", "painting" ] }]`
docs = ${json:json}

aggregate ${object}
    documents docs
    facet categorizedByTage
        unwind "$tags"
        sortByCount "$tags"
    , categorizedByPrice
        match price != null
        bucket "$price" boundaries [0, 150, 200, 300, 400] default "Other" output count=sum(1), titles=push("$title") 
    , "categorizedByYears(Auto)"
        bucketAuto "$year" buckets 4 
    end
end