db={"registration_forms":[{_id:ObjectId("634e9f99f906a707bec7beb1"),date:ISODate("2021-10-18T00:00:00.000Z"),first_name:"Daniel",last_name:"Spenser",staff_name:"john",is_deleted:false,}],"update_forms":[{_id:ObjectId("63831c2d26c1318c7094d473"),date:ISODate("2022-03-11T00:00:00.000Z"),first_name:"Kylee",last_name:"Arroyo",staff_name:"john",is_deleted:false,register_form_id:ObjectId("634e9f99f906a707bec7beb1")},{_id:ObjectId("63831c3f26c1318c7094d485"),date:ISODate("2022-09-15T00:00:00.000Z"),first_name:"Forbes",last_name:"Randall",staff_name:"john",is_deleted:false,register_form_id:ObjectId("634e9f99f906a707bec7beb1")}]}

db.registration_forms.aggregate([{"$match":{_id:ObjectId("634e9f99f906a707bec7beb1"),date:{"$gte":ISODate("2021-10-18T00:00:00.000Z"),"$lte":ISODate("2021-10-20T00:00:00.000Z")},is_deleted:false}},{"$unionWith":{"coll":"update_forms","pipeline":[{$match:{$expr:{$eq:["$register_form_id",ObjectId("634e9f99f906a707bec7beb1")]}}}]}}])