Question

If I have the following 3 documents. How can I select the documents that have at least two purple squares. In this case it would only be the last element.

I know I can select documents with any purple squares with db.foo.find({foo: {"$elemMatch": {shape: "square", color: "purple"}}})

But is there a way to say it must match a certain number of times?

// Document 1
{ "foo" : [
      {
        "shape" : "square",
        "color" : "purple",
        "thick" : false
      },
      {
        "shape" : "circle",
        "color" : "red",
        "thick" : true
      }
] }


// Document 2
{ "foo" : [
      {
        "shape" : "square",
        "color" : "red",
        "thick" : true
      },
      {
        "shape" : "circle",
        "color" : "purple",
        "thick" : false
      }
] }

// Document 3
{ "foo" : [
      {
        "shape" : "square",
        "color" : "purple",
        "thick" : false
      },
      {
        "shape" : "square",
        "color" : "purple",
        "thick" : true
      }
] }

This example is adapted from the last sample here: http://www.mongodb.org/display/DOCS/Dot+Notation+%28Reaching+into+Objects%29

Answer 1

This could be accomplished by using $where, MapReduce, or your application could increment a count of interesting objects when inserted/updated (e.g. db.foo.insert({foo:[{...}, {...}, ...], purpleSquareCount:2});).

The simplest solution is likely to use $where (note the performance implications):

hasPurpleSquares = function () {
    var count = 0;
    this.foo.forEach(function (obj) {
         if (obj.shape == "square" && obj.color == "purple") { 
              count = count + 1;
         }
    });
    if (count >= 2) {
        return true;
    }
}

db.foo.find({$where:hasPurpleSquares});