Display true or false if a user ID exist within an array of objects in an aggregate query

Question

I have this aggregate query returning the title and number of user likes for the post which is working.

const posts = await Post.aggregate([
      { $project:
        {
          title:1, 
          userlikecnt:{$size:"$userlikes"}
        }
      }
    ])

I'm trying to extend it to include if the specific user like that post, then return true/false or if not then a count. This is what I have thus far but it isn't working, any suggestions?

const posts = await Post.aggregate([
      { $project:
        {
          title:1, 
          userlikecnt:{$size:"$userlikes"}, 
          userlikedthisPost:{$sum:{
            $cond:[{$eq:['$userlikes.userid',
               mongoose.Types.ObjectId('64b4717b9c74dbff21f51e0e') ]},1,0] }}
        }
      }
    ])

Answer 1

The $eq check doesn't do what you are looking for here with arrays inside of the aggregation language.

Instead try the $in operator (being sure to flip the order of the arguments). Something like this:

.aggregate([
  {
    $project: {
      title: 1,
      userlikecnt: {
        $size: "$userlikes"
      },
      userlikedthisfilm: {
        $sum: {
          $cond: [
            {
              $in: [
                3,
                "$userlikes.userid"
              ]
            },
            1,
            0
          ]
        }
      }
    }
  }
])

Playground demonstration

Note in the playground I used integers rather than ObjectIds for the userid field of userlikes, be sure to use appropriate types/values for your specific situation