$lookup on ObjectId's in an array

Question

What\'s the syntax for doing a $lookup on a field that is an array of ObjectIds rather than just a single ObjectId?

Example Order Document:

{
  _id:          


        

Answer 1

You can also use the pipeline stage to perform checks on a sub-docunment array

Here's the example using python (sorry I'm snake people).

db.products.aggregate([
  { '$lookup': {
      'from': 'products',
      'let': { 'pid': '$products' },
      'pipeline': [
        { '$match': { '$expr': { '$in': ['$_id', '$$pid'] } } }
        // Add additional stages here 
      ],
      'as':'productObjects'
  }
])

The catch here is to match all objects in the ObjectId array (foreign _id that is in local field/prop products).

You can also clean up or project the foreign records with additional stages, as indicated by the comment above.