Add only a field from another collection in MongoDB

Question

I have two collections

A with schema

{
    a : Array,
    b : ObjectID
}

and B with the following schema

{
    x : 'string',
    y : // some object schema
    ...
    b : ObjectID
}

I want to use mongo aggregate to add a new field in collections B to include only a from collection A searching using b.

I want my value after aggregation to have the following schema :

{
    x : 'string',
    newField : a // array from collection A
    y : // some object schema
    ...
    b : ObjectID
}

only $lookup does not work because i do not want the whole object and also merge will remove _id and merge others which i do not want.

Answer 1

I'm answering this myself after solving it.

This can be done as a pipeline of operations as follows :

{
    $lookup: {
        from: "A",
        localField: "b",
        foreignField: "b",
        as: "someField"
    }
},
{
    $addFields: {
      newField  : "$someField.a"
    }
},
{
    $unwind: "$newField"
},
{
    $project: {
        someField: 0
    }
}

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More efficient answers will be accepted