Querying Nested List Existence in Mongo

Question

I have a document in Mongo that is structured in the following way:

{ 
    "_id" : ObjectId("4eea7237d0ba3a04f20008fb"), 
    "code" : "b2677c2809c844cc9d7e3e4ff8d95b46", 
    "city_id" : 4, 
    "datetime" : ISODate("2011-12-13T18:41:44.062Z"), 
    "plays" : [     
        {   
          "play_id" : 717224,   
          "clicks" : [ ],   
          "order" : 1,  
          "mysql_id" : 145
        }

I want to query for docs whose plays.clicks attribute is a non-empty list. I've tried exists with no luck. I thought that something like this might work:

 db.collection.find({plays.clicks.0: {$exists:true}})

But I believe that this would return only docs whose first element in the plays array contains a non-empty clicks list.

Any thought on how I might get this done?

Thanks

Answer 1

db.collection.find({plays.clicks.0: {$exists:true}})

is the right syntax, however as plays is a list the query will match any document that has clicks in plays. There is no way to retrieve a subset of an Array for subelements in this way[1]. There is a ticket for sub / virtual collections[2]

[1] http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields#RetrievingaSubsetofFields-RetrievingaSubrangeofArrayElements

[2] https://jira.mongodb.org/browse/SERVER-828

Answer 2

Save the size of the list as a separate attribute (e.g. num_plays). Then you can query for documents where num_plays is greater than 0: