gistfile1.py

from mongoengine import Document, StringField

class Machine(Document):
    data_center = StringField(choices=[
        ('us-east', 'US East'),
        ('us-west', 'US West'),
        ('sa-sao', 'South America (SAO)'),
        ('eu', 'European Union'),
    ])

# Connect to a mongo database
from mongoengine import connect
connect('test_map_reduce')

# Now let's create some random records to test map reduce
for x in xrange(0, 1000):
    # Choose one of the data centers in random
    Machine(
        data_center=random.choice(
            ['us-east', 'us-west', 'sa-sao', 'eu']
        )
    ).save()

# Now execute the map reduce
The map function references the variable this to inspect the current object under consideration. See http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-MapFunction
from bson import Code
map = Code("""function () {
    emit(this.data_center, {count: 1});
}""")

# The reduce function
reduce = Code("""function(data_center, record_list) {
    var result = {count: 0};
    record_list.forEach(function (record) {
        result.count += record.count;
    });
    return result;
}""")

result = Machine.objects.map_reduce(map, reduce, 'inline')

for document in result:
    print document.key, document.value