Getting indices of distinct value in an array for each document

user_4832541 0
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I've installed MongoDB and started testing it. I am new to MongoDB and trying to learn it. I have created a collection with the following items:

{ "_id" : ObjectId("5fb60945df88631311ab8a48"), "name" : "x-market", "tags" : ["water", "soda", "water"] }
{ "_id" : ObjectId("5fb60945df88631311ab8a49"), "name" : "y-market", "tags" : ["juice", "water"] }

What I'd like to do is query so I get, for each tag/document, 1 document having the name of the document and the tag with an array of indices of the tag in the tags array. It is complicated but the result should look like this:

{ "_id" : ObjectId, "name" : "x-market", "tag" : "water", index = [0,2]}
{ "_id" : ObjectId, "name" : "x-market", "tag" : "soda", index = [1]}
{ "_id" : ObjectId, "name" : "y-market", "tag" : "juice", index = [0]}
{ "_id" : ObjectId, "name" : "y-market", "tag" : "water", index = [1]}

How do I query for this? I tried to use $group, $project $reduce operators but I got lost and my query is failing to execute. Any help would be appreciated, thank you!

mongodb
1 Answers

You can try,

  • $unwind deconstruct tags array and add arrayindex in index key
  • $group by name and tags, push index in index array, get first name
  • $project to show required fields
db.collection.aggregate([
  {
    $unwind: {
      path: "$tags",
      includeArrayIndex: "index"
    }
  },
  {
    $group: {
      _id: {
        _id: "$_id",
        tags: "$tags"
      },
      name: { $first: "$name" },
      index: { $push: "$index" }
    }
  },
  {
    $project: {
      _id: "$_id._id",
      name: "$name",
      tag: "$_id.tags",
      index: "$index"
    }
  }
])

Playground

user_89871280
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