MongoDB 中的聚合连接不返回结果

Question

I am trying to do an aggregate join to find patient information in the 'patients' table based on 'patient_id' field in the bookings table. Aggregate join is not returning results even though there are matching records in the 'patients' table. Am I doing something wrong?

Answer 1

If the _id field in the "patients" collection is an ObjectId, it won't match when you compare to the string value stored in patient_id . You can use the $addFields aggregation step to add a new field to the documents with the "patient_id" field cast to an ObjectId, but I'd advise you to instead store reference id's as ObjectIds