Use array first field in mongo aggregate $lookup query to match a document

Question

I want to use my array field 0th value to find a match in sale document using Mongo aggregate $lookup query. Here is my query:

db.products.aggregate([ 
{ 
    $match : { _id:ObjectId("57c6957fb190ecc02e8b456b")  }  
},
{
    $lookup : {
        from : 'sale',
        localField: 'categories.0', 
        foreignField: 'saleCategoryId', 
        as : 'pcSales'  
    }
}]);

Result :

{
"_id" : ObjectId("57c6957fb190ecc02e8b456b"),   
"categories" : [
    "57c54f0db190ec430d8b4571"
],  
"pcSales" : [
    {
        "_id" : ObjectId("57c7df5f30fb6eacb3810d1b"),                       
        "Title" : "Latest Arrivals",
    }
]}

This query will return me a match but when i check it not a match. I don't get why is this happening, And when i removed 0th part from query its return blank array. Like this:

{
    "_id" : ObjectId("57c6957fb190ecc02e8b456b"),   
    "categories" : [
        "57c54f0db190ec430d8b4571"
    ],  
    "pcSales" : []
}

saleCategoryId is also a array field which contain array of categoriesKey.

Please help.

Answer 1

Because your localField is an array, you'll need to add an $unwind stage to your pipeline before the lookup or use the $arrayElemAt in a $project pipeline step to get the actual element in the array.

Here are two examples, one which uses the $arrayElemAt operator:

db.products.aggregate([ 
    { "$match" : { "_id": ObjectId("57c6957fb190ecc02e8b456b") } },
    {
        "$project": {
            "category": { "$arrayElemAt": [ "$categories", 0 ] }            
        }
    },
    {
        "$lookup": {
            from : 'sale',
            localField: 'category', 
            foreignField: 'saleCategoryId', 
            as : 'pcSales'  
        }
    }
]);

and this which uses $unwind to flatten the categories array first before applying the $lookup pipeline:

db.products.aggregate([ 
    { "$match" : { "_id": ObjectId("57c6957fb190ecc02e8b456b") } },
    { "$unwind": "$categories" },
    {
        "$lookup": {
            from : 'sale',
            localField: 'categories', 
            foreignField: 'saleCategoryId', 
            as : 'pcSales'  
        }
    }
]);