MongoDb: $sortByCount operator

$sortByCount:

Groups incoming documents based on the value of a specified expression,

then computes the count of documents in each distinct group.

Each output document contains two fields: an _id field containing the

distinct grouping value, and a count field containing the number of

documents belonging to that grouping or category.

The documents are sorted by count in descending order.

db.exhibits.insertMany([

{ "_id" : 1, "title" : "The Pillars of Society", "artist" : "Grosz", "year" : 1926, "tags" : [ "painting", "satire", "Expressionism", "caricature" ] },

{ "_id" : 2, "title" : "Melancholy III", "artist" : "Munch", "year" : 1902, "tags" : [ "woodcut", "Expressionism" ] },

{ "_id" : 3, "title" : "Dancer", "artist" : "Miro", "year" : 1925, "tags" : [ "oil", "Surrealism", "painting" ] },

{ "_id" : 4, "title" : "The Great Wave off Kanagawa", "artist" : "Hokusai", "tags" : [ "woodblock", "ukiyo-e" ] },

{ "_id" : 5, "title" : "The Persistence of Memory", "artist" : "Dali", "year" : 1931, "tags" : [ "Surrealism", "painting", "oil" ] },

{ "_id" : 6, "title" : "Composition VII", "artist" : "Kandinsky", "year" : 1913, "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, "tags" : [ "abstract", "painting" ] }

])

db.exhibits.aggregate( [

{ $unwind: "$tags" }

])

Output:

[

{

_id: 1,

title: 'The Pillars of Society',

artist: 'Grosz',

year: 1926,

tags: 'painting'

},

{

_id: 1,

title: 'The Pillars of Society',

artist: 'Grosz',

year: 1926,

tags: 'satire'

},

{

_id: 1,

title: 'The Pillars of Society',

artist: 'Grosz',

year: 1926,

tags: 'Expressionism'

},

{

_id: 1,

title: 'The Pillars of Society',

artist: 'Grosz',

year: 1926,

tags: 'caricature'

},

{

_id: 2,

title: 'Melancholy III',

artist: 'Munch',

year: 1902,

tags: 'woodcut'

},

{

_id: 2,

title: 'Melancholy III',

artist: 'Munch',

year: 1902,

tags: 'Expressionism'

},

{ _id: 3, title: 'Dancer', artist: 'Miro', year: 1925, tags: 'oil' },

{

_id: 3,

title: 'Dancer',

artist: 'Miro',

year: 1925,

tags: 'Surrealism'

},

{

_id: 3,

title: 'Dancer',

artist: 'Miro',

year: 1925,

tags: 'painting'

},

{

_id: 4,

title: 'The Great Wave off Kanagawa',

artist: 'Hokusai',

tags: 'woodblock'

},

{

_id: 4,

title: 'The Great Wave off Kanagawa',

artist: 'Hokusai',

tags: 'ukiyo-e'

},

{

_id: 5,

title: 'The Persistence of Memory',

artist: 'Dali',

year: 1931,

tags: 'Surrealism'

},

{

_id: 5,

title: 'The Persistence of Memory',

artist: 'Dali',

year: 1931,

tags: 'painting'

},

{

_id: 5,

title: 'The Persistence of Memory',

artist: 'Dali',

year: 1931,

tags: 'oil'

},

{

_id: 6,

title: 'Composition VII',

artist: 'Kandinsky',

year: 1913,

tags: 'oil'

},

{

_id: 6,

title: 'Composition VII',

artist: 'Kandinsky',

year: 1913,

tags: 'painting'

},

{

_id: 6,

title: 'Composition VII',

artist: 'Kandinsky',

year: 1913,

tags: 'abstract'

},

{

_id: 7,

title: 'The Scream',

artist: 'Munch',

year: 1893,

tags: 'Expressionism'

},

{

_id: 7,

title: 'The Scream',

artist: 'Munch',

year: 1893,

tags: 'painting'

},

{

_id: 7,

title: 'The Scream',

artist: 'Munch',

year: 1893,

tags: 'oil'

}

]

Actual query:

Gives count based on tags & sort documents

descending order based on count.

db.exhibits.aggregate( [

{ $unwind: "$tags" },

{ $sortByCount: "$tags" }

])

Output:

[

{ _id: 'painting', count: 6 },

{ _id: 'oil', count: 4 },

{ _id: 'Expressionism', count: 3 },

{ _id: 'abstract', count: 2 },

{ _id: 'Surrealism', count: 2 },

{ _id: 'ukiyo-e', count: 1 },

{ _id: 'woodcut', count: 1 },

{ _id: 'woodblock', count: 1 },

{ _id: 'caricature', count: 1 },

{ _id: 'satire', count: 1 }

]