Searching a MongoDB collection based on another

Question

I have a document called Mapping , which has an _id and an array of objects called Mappings. I have another collection called NewMappings . For each _id in NewMappings , I need to search within the array of Mappings (of Mapping collection) and return the _id of Mapping .

I wrote something like this, but it failed to return anything.

var d=db.NewMappings.find(); 
d.forEach(function(item){
    db.matching.find({Mappings: {$elemMatch : {TargetId: item._id}}})
})

however, this query returned values

var d=db.NewMappings.find(); 
db.matching.find({Mappings: {$elemMatch : {TargetId: d[0]._id}}})

Am I missing something? Please help me. I am in dark. thanks in advance.

Answer 1

One way, if you just want to see the results is:

var d=db.NewMappings.find(); 
d.forEach(function(item){
    db.matching.find({Mappings: {$elemMatch : {TargetId: item._id}}}).forEach(printjson)
})

You can also use the aggregation framework

Answer 2

Sounds like you're trying to do a Join, which isn't explicitly supported in MongoDB. You'll need to either use a few aggregation functions to flatten the data or a mapreduce.

Here's an example using mapreduce to restructure information: http://cookbook.mongodb.org/patterns/pivot/