MongoDB query to find property of first element of array

Question

I have the following data in MongoDB (simplified for what is necessary to my question).

{
    _id: 0,
    actions: [
        {
            type: "insert",
            data: "abc, quite possibly very very large"
        }
    ]
}
{
    _id: 1,
    actions: [
        {
            type: "update",
            data: "def"
        },{
            type: "delete",
            data: "ghi"
        }
    ]
}

What I would like is to find the first action type for each document, e.g.

{_id:0, first_action_type:"insert"}
{_id:1, first_action_type:"update"}

(It's fine if the data structured differently, but I need those values present, somehow.)

EDIT: I've tried db.collection.find({}, {'actions.action_type':1}), but obviously that returns all elements of the actions array.

NoSQL is quite new to me. Before, I would have stored all this in two tables in a relational database and done something like SELECT id, (SELECT type FROM action WHERE document_id = d.id ORDER BY seq LIMIT 1) action_type FROM document d.

These documents are large...I looking to use a projection or something to help me get only what I need. — Paul Draper
– Paul Draper, Commented Oct 4, 2013 at 7:00
In a SO question like this, you should include an attempted solution, in this case a query you have tried, which did not work. — hyde
– hyde, Commented Oct 4, 2013 at 7:10

attish · Accepted Answer · 2013-10-04 13:05:57Z

12

You can use $slice operator in projection. (but for what you do i am not sure that the order of the array remain the same when you update it. Just to keep in mind))

db.collection.find({},{'actions':{$slice:1},'actions.type':1})

answered Oct 4, 2013 at 13:05

attish

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scribu · Accepted Answer · 2020-06-12 12:56:05Z

2

Using the $arrayElemAt operator is actually the most elegant way, although the syntax may be unintuitive:

db.collection.aggregate([
  { $project: {first_action_type: {$arrayElemAt: ["$actions.type", 0]}
])

answered Jun 12, 2020 at 12:56

scribu

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Comments

Agis · Accepted Answer · 2013-10-05 08:49:09Z

1

You can also use the Aggregation Pipeline introduced in version 2.2:

db.collection.aggregate([
  { $unwind: '$actions' },
  { $group: { _id: "$_id", first_action_type: { $first: "$actions.type" } } }
])

edited Oct 5, 2013 at 8:49

answered Oct 4, 2013 at 8:56

Agis

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2 Comments

attish Over a year ago

It will give back the whole array, there is a need for an unwind before the group. like this: db.collection.aggregate([ { $unwind: '$actions' }, { $group: { _id: "$_id", first_action_type: { $first: "$actions.type"} } } ])

Agis Over a year ago

@PaulDraper Sorry had a typo. Fixed, you can try now :)

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