Get posts with n latest comments in MongoDB
If you're using MongoDB 3.6 or higher you can use $lookup with custom pipeline to "join" posts with comments and take 2 most recent ones (using $limit)
db.posts.aggregate([ { $sort: { date: -1 } }, { $limit: 8 }, { $lookup: { from: "comments", let: { postId: "$_id" }, pipeline: [ { $match: { $expr: { $eq: [ "$$postId", "$post_id" ] } } }, { $sort: { date: -1 } }, { $limit: 2 } ], as: "comments" } }])