[{"abc":{"array":[{"_id":"a0","timestamp":"ta0","type":"object"},{"_id":"a1","timestamp":"ta1","type":"object"},{"_id":"a2","timestamp":"ta2","type":"object"},{"_id":"a3","timestamp":"ta3","type":"object"},{"_id":"a4","timestamp":"ta4","type":"object"},{"_id":"a5","timestamp":"ta5","type":"object"}]}},{"abc":{"array":[{"_id":"b0","timestamp":"tb0","type":"object"},{"_id":"b1","timestamp":"tb1","type":"object"},{"_id":"b2","timestamp":"tb2","type":"object"},{"_id":"b3","timestamp":"tb3","type":"object"}]}}]

db.collection.aggregate([{$addFields:{"abc.array":{$map:{input:"$abc.array",as:"element",in:{$mergeObjects:["$$element",{index:{$indexOfArray:["$abc.array","$$element"]}}]}}}}}])