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italktothewind
italktothewind

Reputation: 2195

MongoDb aggregation framework push all values of a field to an array

I want an aggregation to transform an array of documents like this:

[
  {
    "key": "key1"
  },
  {
    "key": "key2"
  }
]

into an array of:

[
  "key1",
  "key2"
]

Thanks!

Upvotes: 1

Views: 2456

Answers (2)

whoami - fakeFaceTrueSoul
whoami - fakeFaceTrueSoul

Reputation: 17915

Issue with your query:

In your query when you're doing { $replaceRoot: { newRoot: "$key" } }, $key will get value of key field i.e; key1 for 1st doc & key2 for 2nd doc. But as $replaceRoot takes in an object and make that object as new doc (Root of the document) it's find a string value rather-than an object, which is why it's failing. Anyhow you need to use $group stage to do this which iterates on all docs (Vs) $replaceRoot stage will be executed on each doc individually.

Putting my comment as answer with added description : When you use aggregation you'll either get an empty array [] if not matching docs found/not docs exists Or else you'll get an arrays of objects/object [{},{}] or [{}] in the result. So you can't get an array of elements in the result like : ['key1','key2'], as I said aggregation result array will have a doc in it, then it will be like [{keys : ['key1','key2']}]. You can get it by using below query :

Query :

db.collection.aggregate([
  /** grouping all docs without any condition in `_id` & pushing all values to an array `keys` (name is just a choice), for unique values use `$addToSet` instead of `$push`. */
  {
    $group: { _id: "", keys: { $push: "$key" } } },
  {
    $project: { _id: 0, keys: 1 } // Optional stage, You need to remove `_id` from result as it's included by default
  }
])

Test : mongoplayground

Ref : aggregation-pipeline

Upvotes: 2

Puneet Singh
Puneet Singh

Reputation: 3543

You can do this by $group operator

db.getCollection('collection_name').aggregate([
  {
    $group: {
      _id: null,
      key: {
        $push: "$key"
      }
    }
  }
])

https://mongoplayground.net/p/NfLonicjkaZ

Upvotes: 1

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