How to Query Referenced Objects in Mongodb

How do I query referenced objects in MongoDB?

You can now do it in Mongo 3.2 using $lookup

$lookup takes four arguments

from: Specifies the collection in the same database to perform the join with. The from collection cannot be sharded.

localField: Specifies the field from the documents input to the $lookup stage. $lookup performs an equality match on the localField to the foreignField from the documents of the from collection.

foreignField: Specifies the field from the documents in the from collection.

as: Specifies the name of the new array field to add to the input documents. The new array field contains the matching documents from the from collection.

db.Foo.aggregate(
  {$unwind: "$bars"},
  {$lookup: {
    from:"bar",
    localField: "bars",
    foreignField: "_id",
    as: "bar"

   }},
   {$match: {
    "bar.testprop": true
   }}
)

How to query by reference field in MongoDB?

You can use below aggregation with mongodb 3.6 and above

db.country.aggregate([
  { "$match": { "name": "VietNam" } },
  { "$lookup": {
    "from": Users.collection.name,
    "let": { "countryId": "$_id" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": [ "$country", "$$countryId" ] } } },
    ],
    "as": "users",
  }},
  { "$unwind": "$users" },
  { "$replaceRoot": { "newRoot": "$users" }}
])

MongoDB query referenced documents

Mongodb provides $lookup operator for joining two collections. Its basically like a join in sql, where you'd do a match between _id in A and activity_creator_id in B.
If you're using mongoose odm you'd directly do a populate in the find query in collection B itself.

How to query to get all documents referenced to another collection based on one field as username

You can use the $lookup aggregation to join collections.

db.profiles.aggregate([
  {
    $match: {
      username: "Jakos"
    }
  },
  {
    $lookup: {
      from: "posts",   //the name of the posts collection, change this if it is different
      localField: "_id",
      foreignField: "username",
      as: "posts"
    }
  },
  {
    $project: {
      username: 1,
      posts: 1,
      postsCount: {
        $size: "$posts"
      },
      _id: 0
    }
  }
])

Playground

For mongoose it should be like this in your app:

const profileWithExperiences = await Student.aggregate([
  { $match: { username: res.username.username } },
  {
    $unwind: "$experience"
  },
  {
    $lookup: {
      from: "posts", //the name of the posts collection, change this if it is different
      localField: "_id",
      foreignField: "username",
      as: "posts"
    }
  },
  {
    $project: {
      username: 1,
      posts: 1,
      postsCount: {
        $size: "$posts"
      },
      _id: 0
    }
  }
]);

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