Database
[{"_id":ObjectId("612370c2abbe776854d8c0ec"),"batch_number":0,"job":ObjectId("612370c1abbe777ae8d8c0eb"),"labellers":[{"labeller":ObjectId("61238ed4abbe77763ed8c100"),"completed":true},{"labeller":ObjectId("61238f2babbe77c838d8c101"),"completed":true}],"__v":0},{"_id":ObjectId("612370c2abbe771a75d8c0ed"),"batch_number":1,"job":ObjectId("612370c1abbe777ae8d8c0eb"),"labellers":[{"labeller":ObjectId("61238ed4abbe77763ed8c100"),"completed":true},{"labeller":ObjectId("61238f2babbe77c838d8c101"),"completed":false}],"__v":0}]
Query
db.collection.aggregate([{$group:{_id:"$job",count:{$sum:{$size:{$filter:{input:"$labellers.completed",cond:"$$this"}}}}}}])