How to use dynamic schema in spring data with mongodb?

First, a few insightful links about schemaless data:

• what does “schemaless” even mean anyway?
• “schemaless” doesn’t mean “schemafree”

Second… one may wonder if Spring, or Java, is the right solution for your problem – why not a more dynamic tool, such a Ruby, Python or the Mongoshell?

That being said, let’s focus on the technical issue.

If your goal is only to store random data, you could basically just define your own controller and use the MongoDB Java Driver directly.

If you really insist on having no predefined schema for your domain object class, use this:

@Document(collection = "users")
public class User implements UserDetails {
    @Id
    private String id;
    private Map<String, Object> schemalessData;

    // getters/setters omitted
}

Basically it gives you a container in which you can put whatever you want, but watch out for serialization/deserialization issues (this may become tricky if you had ObjectIds and DBRefs in your nested document). Also, updating data may become nasty if your data hierarchy becomes too complex.

Still, at some point, you’ll realize your data indeed has a schema that can be pinpointed and put into well-defined POJOs.

Update

A late update since people still happen to read this post in 2020: the Jackson annotations JsonAnyGetter and JsonAnySetter let you hide the root of the schemaless-data container so your unknown fields can be sent as top-level fields in your payload. They will still be stored nested in your MongoDB document, but will appear as top-level fields when the ressource is requested through Spring.

@Document(collection = "users")
public class User implements UserDetails {
    @Id
    private String id;

    // add all other expected fields (getters/setters omitted)
    private String foo;
    private String bar;

    // a container for all unexpected fields
    private Map<String, Object> schemalessData;

    @JsonAnySetter
    public void add(String key, Object value) {
        if (null == schemalessData) {
            schemalessData = new HashMap<>();
        }
        schemalessData.put(key, value);
    }

    @JsonAnyGetter
    public Map<String, Object> get() {
        return schemalessData;
    }

    // getters/setters omitted
}