Aggregation Framework Examples — MongoDB C Driver 1.9.2

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# Aggregation Framework Examples

This document provides a number of practical examples that display the capabilities of the aggregation framework.

The Aggregations using the Zip Codes Data Set examples uses a publicly available data set of all zipcodes and populations in the United States. These data are available at: zips.json.

## Requirements

Let’s check if everything is installed.

Use the following command to load zips.json data set into mongod instance:

    $ mongoimport --drop -d test -c zipcodes zips.json

Let’s use the MongoDB shell to verify that everything was imported successfully.

    $ mongo test
    MongoDB shell version: 2.6.1
    connecting to: test
    > db.zipcodes.count()
    29467
    > db.zipcodes.findOne()
    {
          "_id" : "35004",
          "city" : "ACMAR",
          "loc" : [
                  -86.51557,
                  33.584132
          ],
          "pop" : 6055,
          "state" : "AL"
    }

## Aggregations using the Zip Codes Data Set

Each document in this collection has the following form:

    {
      "_id" : "35004",
      "city" : "Acmar",
      "state" : "AL",
      "pop" : 6055,
      "loc" : [-86.51557, 33.584132]
    }

In these documents:

- The `_id` field holds the zipcode as a string.
- The `city` field holds the city name.
- The `state` field holds the two letter state abbreviation.
- The `pop` field holds the population.
- The `loc` field holds the location as a `[latitude, longitude]` array.

## States with Populations Over 10 Million

To get all states with a population greater than 10 million, use the following aggregation pipeline:

aggregation1.c

    #include <mongoc.h>
    #include <stdio.h>

    static void
    print_pipeline (mongoc_collection_t *collection)
    {
       mongoc_cursor_t *cursor;
       bson_error_t error;
       const bson_t *doc;
       bson_t *pipeline;
       char *str;

       pipeline = BCON_NEW ("pipeline",
                            "[",
                            "{",
                            "$group",
                            "{",
                            "_id",
                            "$state",
                            "total_pop",
                            "{",
                            "$sum",
                            "$pop",
                            "}",
                            "}",
                            "}",
                            "{",
                            "$match",
                            "{",
                            "total_pop",
                            "{",
                            "$gte",
                            BCON_INT32 (10000000),
                            "}",
                            "}",
                            "}",
                            "]");

       cursor = mongoc_collection_aggregate (
          collection, MONGOC_QUERY_NONE, pipeline, NULL, NULL);

       while (mongoc_cursor_next (cursor, &doc)) {
          str = bson_as_canonical_extended_json (doc, NULL);
          printf ("%s\n", str);
          bson_free (str);
       }

       if (mongoc_cursor_error (cursor, &error)) {
          fprintf (stderr, "Cursor Failure: %s\n", error.message);
       }

       mongoc_cursor_destroy (cursor);
       bson_destroy (pipeline);
    }

    int
    main (int argc, char *argv[])
    {
       mongoc_client_t *client;
       mongoc_collection_t *collection;

       mongoc_init ();

       client = mongoc_client_new (
          "mongodb://localhost:27017?appname=aggregation-example");
       mongoc_client_set_error_api (client, 2);
       collection = mongoc_client_get_collection (client, "test", "zipcodes");

       print_pipeline (collection);

       mongoc_collection_destroy (collection);
       mongoc_client_destroy (client);

       mongoc_cleanup ();

       return 0;
    }

You should see a result like the following:

    { "_id" : "PA", "total_pop" : 11881643 }
    { "_id" : "OH", "total_pop" : 10847115 }
    { "_id" : "NY", "total_pop" : 17990455 }
    { "_id" : "FL", "total_pop" : 12937284 }
    { "_id" : "TX", "total_pop" : 16986510 }
    { "_id" : "IL", "total_pop" : 11430472 }
    { "_id" : "CA", "total_pop" : 29760021 }

The above aggregation pipeline is build from two pipeline operators: `$group` and `$match`.

The `$group` pipeline operator requires _id field where we specify grouping; remaining fields specify how to generate composite value and must use one of the group aggregation functions: `$addToSet`, `$first`, `$last`, `$max`, `$min`, `$avg`, `$push`, `$sum`. The `$match` pipeline operator syntax is the same as the read operation query syntax.

The `$group` process reads all documents and for each state it creates a separate document, for example:

    { "_id" : "WA", "total_pop" : 4866692 }

The `total_pop` field uses the $sum aggregation function to sum the values of all pop fields in the source documents.

Documents created by `$group` are piped to the `$match` pipeline operator. It returns the documents with the value of `total_pop` field greater than or equal to 10 million.

## Average City Population by State

To get the first three states with the greatest average population per city, use the following aggregation:

    pipeline = BCON_NEW ("pipeline", "[",
       "{", "$group", "{", "_id", "{", "state", "$state", "city", "$city", "}", "pop", "{", "$sum", "$pop", "}", "}", "}",
       "{", "$group", "{", "_id", "$_id.state", "avg_city_pop", "{", "$avg", "$pop", "}", "}", "}",
       "{", "$sort", "{", "avg_city_pop", BCON_INT32 (-1), "}", "}",
       "{", "$limit", BCON_INT32 (3) "}",
    "]");

This aggregate pipeline produces:

    { "_id" : "DC", "avg_city_pop" : 303450.0 }
    { "_id" : "FL", "avg_city_pop" : 27942.29805615551 }
    { "_id" : "CA", "avg_city_pop" : 27735.341099720412 }

The above aggregation pipeline is build from three pipeline operators: `$group`, `$sort` and `$limit`.

The first `$group` operator creates the following documents:

    { "_id" : { "state" : "WY", "city" : "Smoot" }, "pop" : 414 }

Note, that the `$group` operator can’t use nested documents except the `_id` field.

The second `$group` uses these documents to create the following documents:

    { "_id" : "FL", "avg_city_pop" : 27942.29805615551 }

These documents are sorted by the `avg_city_pop` field in descending order. Finally, the `$limit` pipeline operator returns the first 3 documents from the sorted set.

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