Mongoose query to list the document based on value from nested document
It doesn't work this way because product_items.product_size evaluates to an array of objects and you are trying to compare a single object with such array. It is more reliable to use $elemMatch when working with arrays of objects:
db.collection.aggregate([ { $match: { "product_items": { $elemMatch: { "product_size.value": 22, "product_size.unit": "ml" } } } }])