How to select only first and last element from an array without aggregate?

Question

I want to retrieve only the first and last element from an array in a document, by using the "find" function, without using "aggregate".

Here is a Mongo Playground Link

Consider a document

{
    _id:"1",
    messages:[
        {
            "text":"First item",
        },
        {
            "text":"Middle item",
        },
        {
            "text":"Last item",
        },
    ],
}

So far, I was able to retrieve only the first or last, for example, to get the last item, I am doing:

messages: {
    $slice: ["$messages", -1], //Get the last element
},

How can I get both the first (0th index) and the last (nth index) at the same time?

Note: I do not have any information about the length of the array from beforehand.

Note 2: The reason why I would prefer not using "aggregate" is due to having more filters, selections, limits and other various query properties being handled through the find(), limit(), etc. functions of mongoose, which I am using in a generic way to define my wrapper.

I can't think of a way without aggregation... What are your other filters? Maybe you can migrate them all to aggregation. — Rubén Vega
– Rubén Vega, Commented Nov 3, 2021 at 7:57

Joe · Accepted Answer · 2021-11-03 15:26:23Z

3

If you want both values, list them both with a projection like:

{
  messages: [
    {$slice: ["$messages", 1]},
    {$slice: ["$messages",-1],}
  ],
}

answered Nov 3, 2021 at 15:26

Joe

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1 Comment

Chique Over a year ago

Perfect, this is what I needed.

J.F. · Accepted Answer · 2021-11-02 23:57:52Z

1

Using aggregate you can use $first and $last like this:

db.collection.aggregate([
  {
    "$project": {
      "first": {
        "$first": "$messages"
      },
      "last": {
        "$last": "$messages"
      }
    }
  }
])

Example here

answered Nov 2, 2021 at 23:57

J.F.

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1 Comment

Chique Over a year ago

Thank you. Is it possible to do it by normal find method? I am using various other filters, selections and etc. And in that case it'll be great if I can just modify part of the "select" area, like I am doing in my example playground.

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