Aggregations
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:
datastore.aggregate(Book.class).pipeline(
group(id("author"))
.field("books", push("$title")),
sort()
.ascending("name"))
.execute(Author.class);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 with the group() stage. 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 defining the group ID as the author field which will collect all the books by the author
together.
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 optionally 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.
$out
Depending your use case, you might not want 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.
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Any existing data in the collection will be replaced by the output of the aggregation. |
An example aggregation using the $out stage looks like this:
datastore.aggregate(Book.class).pipeline(
group(id("$author"))
.field("books",
push().single("$title")),
out(Author.class))
.execute();You’ll note that out() is the final stage. $out and $merge must be the final stage in our pipeline. We pass a type to out()
that reflects the collection we want to write our output to. Morphia will use the type-to-collection mapping you’ve defined when mapping
your entities to determine that collection name. You may also pass a String with the collection name as well if the target collection
does not correspond to a mapped entity.
$merge
$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
overwrite 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 $merge might look something like this:
aggregation.pipeline(
group(id()
.field("fiscal_year", "$fiscal_year")
.field("dept", "$dept"))
.field("salaries", sum("$salary")),
merge("reporting", "budgets")
.on("_id")
.whenMatched(REPLACE)
.whenNotMatched(INSERT))
.execute();Much like out() above, for merge() we pass in a collection information but here we are also passing in which database to find/create
the collection in. A merge is slightly more complex and so has more options to consider.
In this example, we’re merging in to the budgets collection in the reporting database 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. Other options are defined on com.mongodb.client.model.MergeOptions type defined by the Java driver.
Supported Operators
Every effort is made to provide 100% coverage of all the operators offered by MongoDB. A select handful of operators have been excluded for reasons of suitability in Morphia. In short, some operators just don’t make sense in Morphia. Below is listed all the currently supported operators. If an operator is missing and you think it should be included, please file an issue for that operator.
| Operator | Docs |
|---|---|
| Operator | Docs |
|---|---|
AccumulatorExpressions#accumulator(String,String,List,String) |
|