[{"key":1,text:"do it!",done:false,owner:"dan",tags:["home"],},{"key":2,text:"do something else",done:false,owner:"mike",tags:["home","work"],},{key:3,text:"fix stuff",done:true,owner:"mike",tags:["home","garden"],},{key:4,text:"eat something",done:false,owner:"mike",tags:[],}]

db.collection.aggregate([{$match:{done:false,}},{$group:{"_id":"$owner","openTasks":{$count:{}}}}])