mongodb results with array to individual results map

Question

I have an aggregate query that is doing exactly what I want up to this point, but I am stuck transforming the data at the last step. I have the following:

{
    "name": "a",
    "tasks": [ { .. }, { .. }]
},
{
    "name": "b",
    "tasks": [ { .. }, { .. }]
}

all I need to do is map each entry in tasks to a separate result. Sounds incredibly simple to me.

The expected output should be

{ "name": "a", "task": { .. } },
{ "name": "a", "task": { .. } },
{ "name": "b", "task": { .. } },
{ "name": "b", "task": { .. } }

I tried using { $unwind: '$tasks' } in hopes that I could get separate entries, but it only seems to expand the first result. I only get the tasks from a.

I also tried $map but had similar problems.

If necessary, there is an _id inside of each task that could be used for uniqueness. Thats really what I'm trying to accomplish, a map of key-value pairs such as

{ "_id": "idFromTheTask", "name": "a" }

mickl · Accepted Answer · 2019-01-10 18:39:36Z

1

$unwind is the right way unless your tasks array is empty. In that case it will remove such document. To prevent that you can use preserveNullAndEmptyArrays, try:

db.collection.aggregate([
    {
        $unwind: {
            path: "$tasks",
            preserveNullAndEmptyArrays: true
        }
    }
])

An example here

answered Jan 10, 2019 at 18:39

mickl

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4 Comments

Felipe Over a year ago

odd. I am getting 20 results, but there are 40 total tasks among 5 original results. Still not sure why

Felipe Over a year ago

Actually, the first entry alone has 21 tasks, and using $unwind is still only giving me 20 in robo 3t

mickl Over a year ago

I think 20 is the default size of the cursor, maybe you can run .toArray on it or just add { limit: 100 } to control the number of returned documents

Felipe Over a year ago

you are right, adding toArray() in robo 3t solved the issue

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