{ "_id" : "G", "name" : "Galway", "pop" : 258058 }
{ "_id" : 3, "car" : { "reg" : "142-G-28", "engineSize" : 1 }, "addresses" : [ "MO", "G", "WH" ] }

db.docs.aggregate([{
         $project:
           {item: 1,
             pop:{
                 $cond: { if: { $gte: [ '$pop', 100000 ] }, then: 'Big county', else: 'Small county'}}}}])

db.docs.find({"pop":{$gte:100000}})

db.docs.find({"pop":{$gte:100000}},{"_id":0, "name":1})