The aggregation framework in MongoDB allows you to define a series (called a pipeline) of operations (called stages) against the data in a collection. These pipelines can be used for analytics or they can be used to convert your data from one form to another. This guide will not go in to the details of how aggregation works, however. The official MongoDB documentation has extensive tutorials on such details. Rather, this guide will focus on the Morphia API. The examples shown here are taken from the tests in Morphia itself.

Writing an aggregation pipeline starts just like writing a standard query. As with querying, we start with the Datastore:

MorphiaCursor<Author> aggregate = getDs().aggregate(Book.class)
                .field("books", push(field("title"))))

aggregate() takes a Class literal. This lets Morphia know which collection to perform this aggregation against. Because of the transformational operations available in the aggregation pipeline, Morphia can not validate as much as it can with querying so care will need to be taken to ensure document fields actually exist when referencing them in your pipeline.

The Pipeline

Aggregation pipelines are comprised of a series stages. Our example here has only one stage: group(). This method is the Morphia equivalent of the $group operator. This stage, as the name suggests, groups together documents based on various criteria. In this example, we are collecting together all the books by author. group() takes one parameter: a Group instance. This instance is created using the Group.of() method which will return a new Group stage instance. There are a few overloads but the simplest version, of(), will generate a $group stage with no _id field in the document. In this case, we’re passing the output of the id(String) method which creates an _id field using the values found in the author properties found in the collection’s documents.

The next step defines a new field, books comprised of the titles of the books found in each document. (For reference, this example is the Morphia equivalent of an example found in the aggregation tutorials.) This results in a series of documents that look like this:

{ "_id" : "Homer", "books" : [ "The Odyssey", "Iliad" ] }
{ "_id" : "Dante", "books" : [ "The Banquet", "Divine Comedy", "Eclogues" ] }

Executing the Pipeline

Once your pipeline is complete, you can execute it via the execute() method. This method takes a Class reference for the target type of your aggregation. Given this type, Morphia will map each document in the results and return it. Additionally, you can also include some options to execute(). We can use the various options on the AggregationOptions class to configure how we want the pipeline to execute.


Depending your use case, you might not watch to return the results of your aggregation but simply output them to another collection. That’s where $out comes in. $out is an operator that allows the results of a pipeline to be stored in to a named collection. This collection can not be sharded or a capped collection, however. This collection, if it does not exist, will be created upon execution of the pipeline.

Any existing data in the collection will be replaced by the output of the aggregation.

An example aggregation using the $out stage looks like this:

                   .field("books", push()

You’ll note that out() is the final method called here rather than execute(). This is because $out must be the final stage in the pipeline and it simply makes no sense to do anything other than execute the pipeline at that point. If you look at what we’re passing to the method, you’ll notice the Out class. There are currently two methods of note on Out: to(Class) and to(String). Using either of these methods instructs Morphia to write to either the collection mapped for the given Class or the named collection as noted by the String give.

You may be wondering about the use of Out here and that it seems a bit overcomplicated. One of the design goals for the 2.0 API is to simplify the overall API and to reduce the number of overloads through the introduction of parameter or options objects. By limiting the Aggregation API to two methods for $out (the seconds takes an AggregationOptions reference), we can keep Aggregation itself slim and incorporate any future variations in $out limited to the Out class. This should, hopefully, make both APIs easier to digest and evolve.


$merge is a very similar option with a some major differences. The biggest difference is that $merge can write to existing collections without destroying the existing documents. $out would obliterate any existing documents and replace them with the results of the pipeline. $merge, however, can deposit these new results alongside existing data and update existing data.

Using $merge might look something like this:

               .field("salaries", sum(field("salary"))))

Much like out() above, for merge() we pass in a Merge reference as created by the Merge.into() method. A merge is slightly more complex and so has more options to consider. In this example, we’re merging in to the budgets collection and merging any existing documents based on the _id as denoted using the on() method. Because there may be existing data in the collection, we need to instruct the operation how to handle those cases. In this example, when documents matching we’re choosing to replace them and when they don’t we’re instructing the operation to insert the new documents in to the collection.