Database Reference
In-Depth Information
In addition to the
key
,
initial
, and
reduce
parameters, you can specify three more
optional parameters:
•
keyf
: You can use this parameter to replace the
key
parameter
if you do not wish to group the results on an existing key in
your documents. Instead, you would group them using another
function you design that specifies how to do grouping.
•
cond
: You can use this parameter to specify an additional
statement that must be true before a document will be grouped.
You can use this much as you use the
find()
query to search
for documents in your collection. If this parameter isn't set (the
default), then all documents in the collection will be checked.
•
finalize
: You can use this parameter to specify a function you
want to execute before the final results are returned. For instance,
you might calculate an average or perform a count and include
this information in the results.
■
the
group()
function does not currently work in sharded environments. For these,
you should use the
mapreduce()
function instead. also, the resulting output cannot contain
more than 10,000 keys in all with the
group()
function, or an exception will be raised.
this too, can be bypassed by using
mapreduce()
.
Note
Working with Conditional Operators
MongoDB supports a large set of conditional operators to better filter your results. The
following sections provide an overview of these operators, including some basic examples
that show you how to use them. Before walking through these examples, however, you
should add a few more items to the database; doing so will let you see the effects of these
operators more plainly:
dvd = ( { "Type" : "DVD", "Title" : "Matrix, The", "Released" : 1999,
"Cast" : ["Keanu Reeves","Carrie-Anne Moss","Laurence Fishburne","Hugo
Weaving","Gloria Foster","Joe Pantoliano"] } )
{
"Type" : "DVD",
"Title" : "Matrix, The",
"Released" : 1999,
"Cast" : [
"Keanu Reeves",
"Carrie-Anne Moss",
"Laurence Fishburne",
