MongoDB $lookup on nested document

in this case there is required a nice play with $unwind and $project in aggregation framework

please see below:

db.alumni.aggregate([
    {$match: {_id: 'john'}},
    {$unwind:"$items"},
    {$unwind:"$items.items"},
    {$lookup: {
        from: 'schools', 
        localField: 'items.items.school', 
        foreignField: '_id', 
        as: 'schoolInfo'}},
    {$unwind:"$schoolInfo"},
    {$project:{
        "_id":1,
        "items":[{
            "name":"$items.name",
            "items":[{
            "school":"$schoolInfo._id"    ,
            "grad":"$items.items.grad"    ,
            "schoolInfo":"$schoolInfo"
            }]
        }]            
    }}
]).pretty()

to see how it works – try removing aggregation stages from query and check document structure.

Would that handle better the multiple schools in the $items.items array?

db.alumni.aggregate([
    {$match: {_id: 'john'}},
    {$unwind:"$items"},
    {$unwind:"$items.items"},
    {$lookup: {
        from: 'schools', 
        localField: 'items.items.school', 
        foreignField: '_id', 
        as: 'schoolInfo'}},
    {$unwind:"$schoolInfo"},
    {$group:{
        _id: {
            _id: '$_id',
            name: '$items.name',
        },
        items: {
            $push: {
                'grad': '$items.items.grad',
                'school': '$schoolInfo._id'
                'schoolInfo': '$schoolInfo'
            }
        }
    }},
    {$group:{
        _id: '$_id._id',
        items: {
            $push: {
                'name': '$_id.name',
                'items': '$items'
            }
        }
    }}
]).pretty()

I didn’t address the case for missing $items.items but you can look at $unwind empty array
Additionally, it’d be better to leave an empty array instead of nothing when there’s no entry, ie have

{
    name: "Johnny",
    items: [],
},