mongodb `$lookup` or `join` with attributes inside array of objects
You need to revert your $map and iterate shaufel_customers instead of customer - this will return two results:
{ $project: { shaufel_customer_union_id: 1, customerUnion: '$customerUnions', shaufel_customers: { $map: { input: "$shaufel_customers", as: "sc", in: { $mergeObjects: [ "$$c", { $arrayElemAt: [{ $filter: { input: "$customers", cond: {$eq: ["$$this._id", "$$sc.customer"]} } }, 0] }, ] } }, } }},