Database
[{"_id":"officeLocation","data":[{"_id":"2233","key":"APAC - ANZ","value":"APAC - ANZ"},{"_id":"21323","key":"APAC - Japan","value":"APAC - Japan"},{"_id":"2221233","key":"EMEA - CE - Benelux - Brussels","value":"EMEA - CE - Benelux - Brussels"},{"_id":"2221233","key":"EMEA - CE - Benelux - London","value":"EMEA - CE - Benelux - London"}]}]
Query
db.collection.aggregate([{"$match":{"_id":"officeLocation"}},{"$unwind":"$data"},{"$set":{"region":{"$first":{"$split":["$data.key"," - "]}}}},{"$group":{"_id":"regions","regions":{"$addToSet":{"_id":new ObjectId(),
"key": "$region",
"value": "$region"
}
}
}
},
{
"$merge": {
"into": "collection",
"on": "_id"
}
}
])