Mongoose aggregation: how do I get a user's top tags?

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Mongoose aggregation: how do I get a user's top tags?

asked 2 months ago

304

For a profile page I want the ten tags a user has posted under most often. Questions store tags as an array of ObjectIds. I can do it with a loop in JS, but that is a lot of round trips — can this be one aggregation?

mongodb

nodejs

javascript

1 Answer

Ada Sharma

• answered 2 months ago

Unwind, group, sort, then look up the names:

1Question.aggregate([
2  { $match: { author: new Types.ObjectId(userId) } },
3  { $unwind: "$tags" },
4  { $group: { _id: "$tags", count: { $sum: 1 } } },
5  { $sort: { count: -1 } },
6  { $limit: 10 },
7  {
8    $lookup: {
9      from: "tags",
10      localField: "_id",
11      foreignField: "_id",
12      as: "tag",
13    },
14  },
15  { $unwind: "$tag" },
16  { $project: { _id: "$tag._id", name: "$tag.name", count: 1 } },
17])

1Question.aggregate([
2  { $match: { author: new Types.ObjectId(userId) } },
3  { $unwind: "$tags" },
4  { $group: { _id: "$tags", count: { $sum: 1 } } },
5  { $sort: { count: -1 } },
6  { $limit: 10 },
7  {
8    $lookup: {
9      from: "tags",
10      localField: "_id",
11      foreignField: "_id",
12      as: "tag",
13    },
14  },
15  { $unwind: "$tag" },
16  { $project: { _id: "$tag._id", name: "$tag.name", count: 1 } },
17])

Sort and limit before the $lookup — otherwise you join every tag the user has ever touched and throw most of it away.

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