I want an aggregation to transform an array of documents like this:
[
{
"key": "key1"
},
{
"key": "key2"
}
]
into an array of:
[
"key1",
"key2"
]
Thanks!
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I tried this without success: mongoplayground.net/p/-r1BuZiLwxUitalktothewind– italktothewind2020-05-20 01:30:45 +00:00Commented May 20, 2020 at 1:30
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Try this : mongoplayground.net/p/xkpGlNi9MwEwhoami - fakeFaceTrueSoul– whoami - fakeFaceTrueSoul2020-05-20 01:33:20 +00:00Commented May 20, 2020 at 1:33
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2 Answers
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
answered May 20, 2020 at 4:14
whoami - fakeFaceTrueSoul
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Comments
You can do this by $group operator
db.getCollection('collection_name').aggregate([
{
$group: {
_id: null,
key: {
$push: "$key"
}
}
}
])
