Database

db={"post":[{"_id":"5f7de8491af5c0e246d42605","title":"Corporate Web Coordinator","body":"...","author":"5f7de8491af5c0e246d42602"}],"comments":[{"_id":"5f7de8491af5c0e246d42609","isHiden":false,"text":"...","post":"5f7de8491af5c0e246d42605"}]}

Query

db.post.aggregate([{"$lookup":{"from":"comments",let:{postID:"$_id"},pipeline:[{$match:{$expr:[{$eq:["$$postID","$post"]}],isHiden:false}}],"as":"comments"}}])

Result