MongoDB deep nested aggregation

Question

tl;dr

What I have:

db.posts.find()

[ 
  { authorId: 1, 
    text: "lorem",
    likes: 22 
    comments: [
      { commentatorId: 1, commentText: "cool"}, 
      { commentatorId: 2, commentText: "nice"}, 
      { commentatorId: 8, commentText: "trash"}
    ]
  }, 
  { authorId: 2, 
    text: "ipsum",
    likes: 33, 
    comments: [
      { commentatorId: 1, commentText: "cool"}, 
      { commentatorId: 1, commentText: "nice"}, 
      { commentatorId: 3, commentText: "trash"}
    ]
  },
  { authorId: 3, 
    text: "ipsum",
    likes: 44, 
    comments: [
      { commentatorId: 2, commentText: "cool"}, 
      { commentatorId: 2, commentText: "nice"}, 
      { commentatorId: 3, commentText: "trash"}
    ]
  },
  { authorId: 1, 
    text: "ipsum",
    likes: 55, 
    comments: [
      { commentatorId: 1, commentText: "cool"}, 
      { commentatorId: 2, commentText: "nice"}, 
      { commentatorId: 3, commentText: "trash"}
    ]
  },
]

What I want to achieve:

{ 
 1: {posts: 2, likes: 77, comments: 4}, 
 2: {posts: 1, likes: 33, comments: 4},
 3: {posts: 1, likes: 44, comments: 3},
 8: {posts: 0, likes: 0,  comments: 1},
}

As you can see, here authorId and commentatorId fields contain an ObjectId key linking to another collection (lets say "users"). Any user can act as author or commentator. Even if user doesn't write any post, he still needs to be included in aggregation result. Number of comments needs to be counted in all posts, not only posts, which authorId equals to commentatorId.

This is not a question. What have you done so far? What is blocking you? — Cid, Jun 07 '18 at 15:07
Doing {$group: {...}} by authorId field I cannot access the data from other posts(which contains comments number for this author) — alex 9835780012, Jun 07 '18 at 15:14

score 0 · Answer 1 · answered Jun 07 '18 at 16:23

I think this is something you want

db.posts.aggregate([
{$unwind:"$comments"},
{$group:{_id:"$authorId", 
"likes":{$addToSet:"$likes"}, "comments":{$addToSet:"$comments"}}},
{$project:{"likes":{$sum:"$likes"}, "posts":{$size:"$likes"}, "comments":{$size:"$comments"}}}
])

MongoDB deep nested aggregation

1 Answers1