Multikeys

MongoDB provides an interesting "multikey" feature that can automatically index arrays of an object's values. A good example is tagging. Suppose you have an article tagged with some category names:

$ dbshell
> db.articles.save( { name: "Warm Weather", author: "Steve", 
                      tags: ['weather', 'hot', 'record', 'april'] } )
> db.articles.find()
{"name" : "Warm Weather" , "author" : "Steve" , 
 "tags" : ["weather","hot","record","april"] , "_id" : "497ce4051ca9ca6d3efca323"}

We can easily perform a query looking for a particular value in the tags array:

> db.articles.find( { tags: 'april' } )
{"name" : "Warm Weather" , "author" : "Steve" , 
 "tags" : ["weather","hot","record","april"] , "_id" : "497ce4051ca9ca6d3efca323"}

Further, we can index on the tags array. Creating an index on an array element indexes results in the database indexing each element of the array:

> db.articles.ensureIndex( { tags : 1 } )
true
> db.articles.find( { tags: 'april' } )
{"name" : "Warm Weather" , "author" : "Steve" , 
 "tags" : ["weather","hot","record","april"] , "_id" : "497ce4051ca9ca6d3efca323"}
> db.articles.find( { tags: 'april' } ).explain()
{"cursor" : "BtreeCursor tags_1" , "startKey" : {"tags" : "april"} , 
 "endKey" : {"tags" : "april"} , "nscanned" : 1 , "n" : 1 , "millis" : 0 }

Embedded object fields in an array

Additionally the same technique can be used for fields in embedded objects:

> db.posts.find( { "comments.author" : "julie" } )
{"title" : "How the west was won" , 
 "comments" : [{"text" : "great!" , "author" : "sam"},
               {"text" : "ok" , "author" : "julie"}],
 "_id" : "497ce79f1ca9ca6d3efca325"}

Querying on all values in a given set

By using the $all query option, a set of values may be supplied each of which must be present in a matching object field. For example:

> db.articles.find( { tags: { $all: [ 'april', 'record' ] } } )
{"name" : "Warm Weather" , "author" : "Steve" , 
 "tags" : ["weather","hot","record","april"] , "_id" : "497ce4051ca9ca6d3efca323"}
> db.articles.find( { tags: { $all: [ 'april', 'june' ] } } )
> // no matches

Multikeys

Embedded object fields in an array

Querying on all values in a given set

See Also

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