MongoDB: Conditional select from one collection based on another collection

Question

I\'m fairly new to MongoDB and need help doing a select, or perhaps some sort of left join, on one collection based on another collection\'s data.

I have two collections

Answer 1

You can try below aggregation query.

db.animals.aggregate([ [
  {
    "$lookup": {
      "from": "meals",
      "localField": "lastMeal",
      "foreignField": "id",
      "as": "last_meal"
    }
  },
  {
    "$unwind": "$last_meal"
  },
  {
    "$match": {
      "last_meal.created": {
        "$gt": 20171001
      }
    }
  }
])

More info here.

You can use $project with exclusion after $match stage to format the response to exclude joined fields. Something like { $project: {"last_meal":0} }