How can i compare two localField with same form in mongoDB

Question

i'm new to mongoBD. i have join two table using $lookup. i have two models Posts and Savedpost. I need to compare two field postId and savedBy i check mongoDb documentation for lookup and i try

Posts.aggregate([{
        $lookup: {
            from: Savedpost.collection.name, // collection name in db
            localField: "_id", // Post id from Post collection
            foreignField: "postId", // postId from Savedpost collection
            as: "isSaved"
        },
    }]).exec(function(err, students) {
        res.send(students)
    });

here i compare with only postId but i need to compare both fields postId and savedBy it is possible and how ?

Thank you

score 3 · Accepted Answer · answered Mar 26 '22 at 20:25

Here is how to $lookup based on two fields:

 db.posts.aggregate([
 {
 $lookup: {
   from: "savedposts",
   let: {
     first: "$_id",
     second: "$savedby"
   },
   pipeline: [
    {
      $match: {
        $expr: {
          $and: [
            {
              $eq: [
                "$postid",
                "$$first"
              ]
            },
            {
              $eq: [
                "$saved",
                "$$second"
              ]
            }
          ]
        }
      }
    }
  ],
  as: "result"
 }
}
])

Explained:

Add two temp variables from the post collection via let
In the lookup pipeline stage match based on the temporary variables to the foreign collection lookup keys.

In the example:

first --> posts._id-->savedposts.postid

second --> posts.savedby-->savedposts.saved

playground

How can i compare two localField with same form in mongoDB

1 Answers1