MongoDb: aggregation $lookup with filtering over the foreign documents

Question

Given a couple of collecitons:

1.- USERS

{
  name: ...
  id: ...
  city: ...
  age: ...
  otherdata: ...
}

2.- PETS

{
          


        

Answer 1

You can use $filter array aggregation operator on pets array that is produced by your $lookup stage.

To output pets older than 1 year use

db.users.aggregate([ 
{ 
  $lookup: 
  { 
    from: "pets", 
    localField: "id", 
    foreignField: "owner", 
    as: "pets" 
  } 
}, 
{
  $project: 
  {
    name: 1,
    pets: 
    { 
      $filter: 
      { 
        input: "$pets", 
        as: "pet", 
        cond: { $gte: [ "$$pet.age", 1 ] } 
      } 
    } 
  } 
} 
]);

To output the oldest pets simply replace cond field of $filter operator in the previous aggregation pipeline with

cond: { $eq: [ "$$pet.age", { $max: "$pets.age" } ] }