mongodb queries find total number of cities in the database - mongodb

Question

Hi everyone I have a huge data that contains some information like this below:
{ "_id" : "01011", "city" : "CHESTER", "loc" : [ -72.988761, 42.279421 ], "pop" : 1688, "state" : "MA" }
{ "_id" : "01012", "city" : "CHESTERFIELD", "loc" : [ -72.833309, 42.38167 ], "pop" : 177, "state" : "MA" }
{ "_id" : "01013", "city" : "CHICOPEE", "loc" : [ -72.607962, 42.162046 ], "pop" : 23396, "state" : "MA" }
{ "_id" : "01020", "city" : "CHICOPEE", "loc" : [ -72.576142, 42.176443 ], "pop" : 31495, "state" : "MA" }
I want to be able to find the number of the cities in this database using Mongodb command. But also the database may have more than one recored that has the same city. As the example above.
I tried:
>db.zipcodes.distinct("city").count();
2015-04-25T15:57:45.446-0400 E QUERY warning: log line attempted (159k) over max size (10k), printing beginning and end ... TypeError: Object AGAWAM,BELCHERTOWN ***data*** has no method 'count'
but I didn't work with me.Also I did something like this:
>db.zipcodes.find({city:.*}).count();
2015-04-25T16:00:01.043-0400 E QUERY SyntaxError: Unexpected token .
But it didn't work also and even if does work it will count the redundant data (city). Any idea?

Answer 1

Instead of doing
db.zipcodes.distinct("city").count();
do this:
db.zipcodes.distinct("city").length;
and there is aggregate function, which may help you.
I have also found 1 example on aggregate (related to your query).
If you want to add condition, then you could refer $gte / $gte (aggregation) and/or $lte / $lte (aggregation)
See, if that helps.

Answer 2

You can also use the aggregation framework for this. The aggregation pipeline has two $group operator stages; the first groups the documents by city and the last calculates the total distinct documents from the previous stream:
db.collection.aggregate([
{
"$group": {
"_id": "$city"
}
},
{
"$group": {
"_id": 0,
"count": { "$sum": 1 }
}
}
]);
Output:
/* 1 */
{
"result" : [
{
"_id" : 0,
"count" : 3
}
],
"ok" : 1
}