将mongoDB $ lookup与$ project结合使用

Question

Right now I'm using the $project aggregation for filtering out unnecessary fields. Also I'm using the $lookup aggregation to link two collections togethe and I know how to use both of them in the main collection. Now my question is; how can I put this $project aggregation inside of a lookup? What I have now is looks like this:

[
            '$lookup' => [
                'from' => Media::collectionName(),
                'localField' => '_id',
                'foreignField' => 'project_id',
                'as' => 'mediaList'
            ]
        ],
        [
            '$project' => [
                'title' => 1,
                'owner_id' => 1,
                'owner_name' => 1,
                'created_at' => 1,
                'updated_at' => 1,
                'status' => 1,
                'discount' => 1,
                'company' => 1,
                'media' => [
                    '$filter' => [
                        'input' => '$mediaList',
                        'as' => 'media',
                        'cond' => $mediaFilter
                    ]
                ]
            ]
        ],

So I can filtering out the unnecessary fields in the main collection. How can I do this in the sub-collection?

Answer 1

If I understood your question correctly, you want to apply a $filter operation on your $mediaList field.

To avoid a redundant $project stage (where you have to declare every single field you want to keep), use $addFields stage instead, like this :

    [
        '$lookup' => [
            'from' => Media::collectionName(),
            'localField' => '_id',
            'foreignField' => 'project_id',
            'as' => 'media' // Note that I use the same name for the field
        ]
    ],
    [
        '$addFields' => [
            'media' => [
                '$filter' => [
                    'input' => '$media',
                    'as' => 'media',
                    'cond' => $mediaFilter
                ]
            ]
        ]
    ],