Spring Data MongoDB - Spring Web Services - Parent - SpringSource

please define productname in your docbook file!Map-Reduce. Using the group operations does have some limitations, for example it is not supportedin a shareded environment and it returns the full result set in a single BSON object, so the result shouldbe small, less than 10,000 keys.Spring provides integration with MongoDB's group operation by providing methods on MongoOperationsto simplify the creation and execution of group operations. It can convert the results of the groupoperation to a POJO and also integrates with Spring's Resource abstraction abstraction. This willlet you place your JavaScript files on the file system, classpath, http server or any other SpringResource implementation and then reference the JavaScript resources via an easy URI style syntax,e.g. 'classpath:reduce.js;. Externalizing JavaScript code in files if often preferable to embedding themas Java strings in your code. Note that you can still pass JavaScript code as Java strings if you prefer.Example UsageIn order to understand how group operations work the following example is used, which is somewhatartifical. For a more realistic example consult the book 'MongoDB - The definitive guide'. A collectionnamed "group_test_collection" created with the following rows.{ "_id" : ObjectId("4ec1d25d41421e2015da64f1"), "x" : 1 }{ "_id" : ObjectId("4ec1d25d41421e2015da64f2"), "x" : 1 }{ "_id" : ObjectId("4ec1d25d41421e2015da64f3"), "x" : 2 }{ "_id" : ObjectId("4ec1d25d41421e2015da64f4"), "x" : 3 }{ "_id" : ObjectId("4ec1d25d41421e2015da64f5"), "x" : 3 }{ "_id" : ObjectId("4ec1d25d41421e2015da64f6"), "x" : 3 }We would like to group by the only field in each row, the 'x' field and aggregate the number of timeseach specific value of 'x' occurs. To do this we need to create an initial document that contains our countvariable and also a reduce function which will increment it each time it is encountered. The Java codeto execute the group operation is shown belowGroupByResults results = mongoTemplate.group("group_test_collection",GroupBy.key("x").initialDocument("{ count: 0 }").reduceFunction("function(doc, prev){ prev.count += 1 }"),XObject.class);The first argument is the name of the collection to run the group operation over, the second is a fluentAPI that specifies properties of the group operation via a GroupBy class. In this example we are usingjust the intialDocument and reduceFunction methods. You can also specify a key-function, aswell as a finalizer as part of the fluent API. If you have multiple keys to group by, you can pass in acomma separated list of keys.The raw results of the group operation is a JSON document that looks like this{}"retval" : [ { "x" : 1.0 , "count" : 2.0} ,{ "x" : 2.0 , "count" : 1.0} ,{ "x" : 3.0 , "count" : 3.0} ] ,"count" : 6.0 ,"keys" : 3 ,"ok" : 1.0The document under the "retval" field is mapped onto the third argument in the group method, in thiscase XObject which is shown below.1.4.0.BUILD-SNAPSHOTSpring Data MongoDB -Reference Documentation 52