Conditional $lookup

Working with Data
18

For anyone coming here, $unionWith is the way to go.
To use the original question’s context, in your aggregation, initially match the records where fruit_name is null, then union that with the records where fruit_name is defined, and perform the lookup operation within the unionWith pipeline. Something like this:

db.child_fruit.aggregate([
  { $match: { fruit_name: null }},
  { $unionWith: {
    coll: "child_fruit",
    pipeline: [
      { $match: { fruit_name: { $ne: null }}},
      { $lookup: {
        from: "fruit",
        ...
      }}
    ]
  },
  ...
]);