Database
[{"original_id":33,"keywords":["dog","cat","goat","dog"]},{"original_id":34,"keywords":["dog","kitten","goat","moose"]},{"original_id":35,"keywords":["moose","elk"]}]
Query
db.collection.aggregate([{$unwind:"$keywords"},{$group:{_id:"$keywords",count:{$sum:1}}},{$group:{_id:null,keywords:{$push:{k:"$_id",v:"$count"}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$keywords"}}}])