[{"title":"fruit mix 1","fruit_details":[{"fruit_names":["strawberry","banana"],"category":"berries"},{"fruit_names":["strawberry","apple","mango"],"category":"red",}]},{"title":"fruit mix 2","fruit_details":[{"fruit_names":["banana","mango"],"category":"tropical",}]},{"title":"fruit mix 3","fruit_details":[{"fruit_names":["banana","lemon"],"category":"yellow",},{"fruit_names":["banana"],"category":"long",},]},]

db.collection.aggregate([{$project:{fruit:{"$reduce":{"input":"$fruit_details","initialValue":[],"in":{"$setUnion":["$$value","$$this.fruit_names",]}}}}},{$unwind:"$fruit"},{$group:{_id:"$fruit",count:{$sum:1,},}},{$sort:{count:-1,}}])