MongoDB Query Embedded Document Field

Question

My collection structure is like in the picture below:

I'm trying to query the field "name" inside every engine:

db.getCollection('scan').find({},
   {
        "engines": {
            "$elemMatch": {
                "name": 1
            }
        }
    }
)

but the returning results contain only the "_id":

Does anyone know why? Thanks!

Answer 1

I'm trying to query the field "name" inside every engine:

Assuming this data sample:

db.collection.insert([
  { engines: { ahnlab: { name: "x", value: "1" }}},
  { engines: { ahnlab: { name: "y", value: "2" }}},
])

You can query all embedded fields name using the dot-notation in the projection operator:

> db.collection.find({},{"engines.ahnlab.name": 1, "_id":0 })
{ "engines" : { "ahnlab" : { "name" : "x" } } }
{ "engines" : { "ahnlab" : { "name" : "y" } } }

• "engines.ahnlab.name": 1 will instruct MongoDB to keep (1) the embedded name field;
• "_id": 0 will instruct MongoDB to not keep the _id field.

---

If you need your output as a flat data structure, you should use the aggregation framework and the $project operator to rewrite your documents:

> db.collection.aggregate([
    {$project: { name: "$engines.ahnlab.name", _id: 0 }}
])
{ "name" : "x" }
{ "name" : "y" }