find in mongoDB documents with filled nested array and build new result object

Question

I have some document with this structure - the category field can also be empty

{
    "_id" : "1",
    "category" : [
        { "target" : "element 1" },
        { "target" : "element 2" }
    ]
},
{
    "_id" : "2",
    "category" : [
        { }
    ]
},
{
    "_id" : "3",
    "category" : [
        { "target" : "element 1" },
        { "target" : "element 2" }
    ]
},

What I need is a result like

{ id: 1, element: "element 1" },
{ id: 1, element: "element 2" },
{ id: 3, element: "element 1" },
{ id: 3, element: "element 2" }

That means first I have to check for all documents which have target elements:

var result = db.collection.find({}, { 'category.target': 1})

1) But this doesn't work, as I still get the empty doc (2).

2) Second I need to get a new result element for each element in the target-array. I think aggregate isn't the correct way as there has to be new objects for each element. And map is only to nave the fields.

So I think I have to iterate over the results and build a new object, correct?

result.forEach(function($res){
    // do a new foreach for every element in target-array?
    newResult.push({ id: res._id, element: res.element });
});

Answer 1

You can do this usingaggregate which, unlike find , allows you to reshape documents.

db.test.aggregate([
    // Duplicate each doc, once per category element
    {$unwind: '$category'},
    // Reshape the documents to create the element field
    {$project: {element: '$category.target'}},
    // Only include docs with an element
    {$match: {element: {$exists: true}}}
])

Result:

"result" : [ 
    {
        "_id" : "1",
        "element" : "element 1"
    }, 
    {
        "_id" : "1",
        "element" : "element 2"
    }, 
    {
        "_id" : "3",
        "element" : "element 1"
    }, 
    {
        "_id" : "3",
        "element" : "element 2"
    }
]