In MongoDB how do you index an embedded object fields in an array?

Question

The mongodb documentation for multikeys gives an example of querying embedded object fields in an array:

http://www.mongodb.org/display/DOCS/Multikeys

But there's no explanation on how you create an index for that situation. Creating an index on the array doesn't seem to work (using the explain mechanism you can see the index isn't use).

Additional details:

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

If you do db.articles.ensureIndex( { comments : 1 } ) it won't index the subfields of the comments objects but rather only the comments object itself.

So the following would use the index:

 > db.posts.find( {comments : { "author" : "julie", "text" : "ok" } } )

Because it's search on the comments objects

But the following wouldn't use the index:

 > db.posts.find( { "comments.author" : "julie" } )

So how do you get mongodb to index for the second case?

As far as I can tell, it does tell you how to add this index: db.articles.ensureIndex( { tags : 1 } ). — Ben Lee
– Ben Lee, Commented Jan 30, 2012 at 12:04

Remon van Vliet · Accepted Answer · 2012-01-30 16:08:45Z

54

You can create the following index :

db.posts.ensureIndex({"comments.author" : 1})

This will index only the author field of the embedded documents. Note that the index will be used for

db.posts.find( { "comments.author" : "julie" } )

As well as

db.posts.find( { comments: {$elemMatch: {author : "julie" }}} )

answered Jan 30, 2012 at 16:08

Remon van Vliet

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2 Comments

jdi Over a year ago

Yes. I think the key point here that the OP and the other answer missed is that the index is dot seperated and contained in double quotes.

unflores Over a year ago

@jdi If I were to find a comment on a post and then run createIndex({"author": 1}) on it would that give the same result as runing db.posts.ensureIndex({"comments.author" : 1})?

crifan · Accepted Answer · 2021-08-16 07:02:15Z

2

now is 2021 year. according to latest Mongodb official doc, should use createIndex：

db.posts.createIndex({"comments.author" : 1})

answered Aug 16, 2021 at 7:02

crifan

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Comments

japrescott · Accepted Answer · 2012-01-30 12:08:22Z

-2

you create the index as if you would with a "normal" field;

db.[collection].ensureIndex( { [yourArrayField] : 1 } )

answered Jan 30, 2012 at 12:08

japrescott

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2 Comments

Imran Over a year ago

That will only index the objects in the array and not the fields on the objects, I've added some extra details into the question to demonstrate.

Jimmy Over a year ago

If the array contains simple string instead of complex object, @japrescott is correct

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