[{"a":1,"b":[{"this":"A","that":"B","other":"C"}],"c":2},{"a":3,"b":[{"this":"D","that":"E","other":"F"},{"this":"G","that":"H","other":"I"}],"c":4}]

db.collection.aggregate([{$addFields:{"b":{$map:{input:"$b",in:{"foo":"$$this","bar":{this_value:"$$this.this"}}}}}}])