MongoDB: query against value in nested array?

Question

I have documents with such kind of schema:

{
   ...
  "coverages" : [
    [
      "GB",
      "WC1"
    ],
    [
      "GB",
      "SE2"
    ],
    ...
  ]
}

I want to find all documents where 'coverages' contains an array second element of which starts with 'WC', i.e. coverages matches with ['GB', /^WC/].

Is there any way to do it with standard mongodb query language?

I guess it could be easily done with custom JavaScript function, but docs says it's much slower...

Thanks!

P.S. I'd like to avoid changing the schema.

update: there is relates bug/misfeture in Mongo: https://jira.mongodb.org/browse/SERVER-1264

Are you aware that the coverages key contains a list of lists? Just want to make sure you are doing that on purpose. — Tyler Brock
– Tyler Brock, Commented Dec 27, 2011 at 16:21
My bad, I was lazy and didn't mentioned that coverages is long list of pairs =\ — Alexander
– Alexander, Commented Dec 28, 2011 at 15:48

Ninja · Accepted Answer · 2012-01-01 15:45:22Z

5

I guess this should work:

db.mycollection.find({"coverages": {"$elemMatch": {"2": /^WC/}}})

edited Jan 1, 2012 at 15:45

answered Dec 27, 2011 at 16:37

Ninja

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4 Comments

Ninja Over a year ago

Just noticed that you have given "coverages" as an array of arrays. Is that a typo or is that how it is? If that is how your schema is defined, then change query to "coverages.0.1"

Alexander Over a year ago

Yes, actually it's long array of arrays (pairs). And I want to find out if it's possible to query it without changing the schema, that's what this question is about :)

Ninja Over a year ago

I've edited the code based on the edited example you have added

matus Over a year ago

what "2" means in the query? Is it interpreted as position in array?

Tyler Brock · Accepted Answer · 2011-12-27 16:28:16Z

0

db.collection.find({ "coverages.0.1": /^WC/ }) should do the trick. It is saying, match documents where the second element of the first array in the key coverages starts with WC.

However, this is a bad schema if you are trying to associate GB (the first item in the array) with the second item in the array.

This is a natural case for the document/object/dictionary which maps keys to values.

Make it like this:

{
  "coverages" : { "GB": "WC1" }
}

Now your query much more naturally matches your logic: db.collection.find({ "coverages.gb": /^WC/ })

answered Dec 27, 2011 at 16:28

Tyler Brock

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1 Comment

Alexander Over a year ago

I've edited example: coverages contains much more than one element, so I guess your approach woudn't work :( I believe natural mongo schema for this case should look like "coverages": [{"country": "GB", "region": "WC1"}, ...].

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