Use array first field in mongo aggregate $lookup query to match a document
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' } }]);