[{"diagnostics":{"_ID":"554bbf7b761e06f02fef3561","tests":[{"_id":"59d678064e4645ec562a37e2","name":"RBC"},{"_id":"59d678064e4645ec562a37e1","name":"Calcium"},{"_id":"59d678064e4645ec562a37e5","name":"Calcium"}]}},{"diagnostics":{"_ID":"554bbf7b761e06f02fef3562","tests":[{"_id":"59d678064e4645ec562a37e2","name":"RBC"},{"_id":"59d678064e4645ec562a37e1","name":"Calcium"}]}}]

db.collection.aggregate([{"$unwind":"$diagnostics.tests"},{"$group":{"_id":{"_id":"$diagnostics.tests.name","diagnosticID":"$diagnostics._ID"},"count":{"$sum":1}}},{"$group":{_id:{"_ID":"$_id.diagnosticID"},"tests":{"$push":{"name":"$_id._id","count":"$count"}}}},{"$project":{"diagnostics._ID":"$_id._ID","diagnostics.tests":"$tests","_id":0,"testCount":{"$size":"$tests"}}}])