How to use $elemMatch on aggregate’s projection?

Quite on old question but literally none of the proposed answers are good.

TLDR:

You can’t use $elemMatch in a $project stage. but you can achieve the same result using other aggregation operators like $filter.

db.itens.aggregate([
    {
        $project: {
            compList: {
               $filter: {
                input: "$complist",
                as: "item",
                cond: {$eq: ["$$item.a", 1]}
               }
            }
        }
    }
])

And if you want just the first item from the array that matches the condition similarly to what $elemMatch does you can incorporate $arrayElemAt

In Depth Explanation:

First let’s understand $elemMatch:

$elemMatch is a query expressions while also this projection version of it exists this refers to a query projection and not $project aggregation stage.

So what? what does this have to do with anything? well a $project stage has certain input structure it can have while the one we want to use is:

<field>: <expression>

What is a valid expression?

Expressions can include field paths, literals, system variables, expression objects, and expression operators. Expressions can be nested.

So we want to use an expression operator, but as you can see from the doc’s $elemMatch is not part of it. hence it’s not a valid expression to be used in an aggregation $project stage.