Spring Data Reactive MongoDB — Aggregation Pipeline

In this article, I would like to show you the Mongo aggregation pipeline using Spring Data Reactive MongoRepository and Reactive MongoTemplate.

Project Setup:

Lets first create a simple spring boot project with these dependencies.

Sample Application:

Let’s consider an application in which freelancers register themselves with their skill sets. The information is stored in the mongoDB as shown below.

"name": "sam",
"age": 40,
"skills": [ "js", "react", "python"]

"name": "jack",
"age": 38,
"skills": [ "js", "angular", "postgres"]

"name": "james",
"age": 30,
"skills": [ "java", "reactor", "mongo"]

"name": "smith",
"age": 32,
"skills": [ "qa", "selenium"]

Users of the application would like to see for each skill set/technology, all the available freelancers. So the UI expectation is more or less like this.


Let’s see how to achieve this using Spring Data.

MongoDB Setup:

I use docker-compose to set up mongodb and insert above documents.

version: "3"
image: mongo
- 27017:27017
image: mongo-express
- 8081:8081

Aggregation Pipeline:

To get the data in the format we want, we need to have following stages.

  • unwind: Each freelancer has an array of skill sets. It is stored in the skills field. This stage reconstructs the document in a way that a person with 3 skills would be presented as 3 persons with 1 skill each.
"$unwind" : {
"path" : "$skills"
  • group: Once you have unwinded (!), now we could group by skills field. For each skill, extract the names and push it to an array!
  • project: Once we have grouped the records, then we need to look for the specific fields we are interested in. I exclude the _id and retrieve as skill, then I also need the names field.

Now let’s get this implemented.

Entity Class:

Our entity class is represented as shown below.

public class Freelancer {

private String id;
private String name;
private int age;
private List<String> skills;


Projection Class:

Since we would like to present the data differently, Lets create a model class for that.

public class SkilledPeople {

private String skill;
private List<String> names;



Mongo Reactive Repository:

Once we have our stages ready, we include them as an array of stages as value for the @Aggregation. Do note that it returns the instances of projection class we have defined above.

Application Properties:



At this point, everything seems to be ready! We can run and verify.


We get the list of skills and corresponding freelancers who can be hired.

SkilledPeople(skill=js, names=[sam, jack])
SkilledPeople(skill=angular, names=[jack])
SkilledPeople(skill=postgres, names=[jack])
SkilledPeople(skill=selenium, names=[smith])
SkilledPeople(skill=java, names=[james])
SkilledPeople(skill=mongo, names=[james])
SkilledPeople(skill=react, names=[sam])
SkilledPeople(skill=python, names=[sam])
SkilledPeople(skill=reactor, names=[james])
SkilledPeople(skill=qa, names=[smith])


Reactive Mongo Template + Aggregation Operation:

Spring Data provides the support for the Mongo aggregation framework. We can create all the stages as shown here and get it executed via Mongo template.


We would get the same output, If we modify the runner as shown below.


We were able to successfully demonstrate the mongo aggregation pipeline using both Spring Reactive Mongo Repository and Mongo Template.

The source code is available here.

Happy learning 🙂




Principal Software Engineer — passionate about software architectural design, microservices.

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Vinoth Selvaraj

Vinoth Selvaraj

Principal Software Engineer — passionate about software architectural design, microservices.

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