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RESTHeart 3.0 is the latest stable version.

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Transformers allow to change the json document representing the resource state, for instance adding, modifying or filtering out some properties.

Resources data flows in and out of RESTHeart in HAL+json format (see Representation Format section), where the state is represented as a json document.

A transformer can be applied either in the REQUEST or RESPONSE phases:

  • Applying it in the REQUEST phase makes sense only for PUT, POST and PATCH requests (when data is sent from the client). In these cases, the incoming data is first transformed and then stored in MongoDB
  • Applying it in the RESPONSE phase, makes sense only for GET requests (when data is sent back to clients). The data is first retrieved from MongoDB, than transformed and passed back to the client.

The rts collection metadata

In RESTHeart, not only documents but also dbs and collections have properties. Some properties are metadata, i.e. they have a special meaning for RESTheart that influences its behavior.

The metadata property rts allows to declare a transformer. The transformer can apply either to the resource itself or its children (for instance, a transformer declared to a collection resource, can be applied to the collection resource itself or to its documents) and to requests or responses.

The metadata property rts is an array of transformer objects. A transformer object has the following format:

{"name":<name>, "phase":<phase>, "scope":<scope>, "args":<args>}


The name of the transformer to apply.


Defines if the transformer is to be applied to (PUT, POST, PATCH) requests or to GET responses

Valid values are "REQUEST" or "RESPONSE".


Defines if the transformer is to be applied to the resource where it is defined or its children.


argsarguments to be passed to the transformer.No

Available and Custom Transformers

RESTHeart comes with 4 ready-to-be-used transformers:

  • addRequestProperties to add properties.
  • filterProperties to filter out properties.
  • stringsToOids and oidsToStrings to convert strings to ObjectIds and vice versa (usually from requests and responses the respectively).

Custom transformers can also be implemented.

Inject properties with addRequestProperties transformer

addRequestProperties is a transformer shipped with RESTHeart that allows to add some properties to the resource state.

The properties are specified via the args property of the transformer object. It is mainly intended to be applied to REQUESTs.

These properties are injected server side. If we need to store some information (such as the username) and we cannot rely on the client, this transformer is the solution.

An example is a blog application where each post document has the author property. This property could be valued server side via this transformer to avoid users to publish posts under a fake identity.

The properties that can be added are:

  • userName
  • userRoles
  • dateTime
  • localIp
  • localPort
  • localServerName
  • queryString
  • relativePath
  • remoteIp
  • requestMethod
  • requestProtocol

For instance, the following request creates the collection rtsexample with this transformer applying to documents in the REQUEST phase. Looking at the args argument we can figure out that the request json data will be transformed adding the log object with the username of authenticated user and its remote ip.

$ http -a a:a PUT rts:='[{"name":"addRequestProperties","phase":"REQUEST","scope":"CHILDREN","args":{"log": ["userName","remoteIp"]}}]'

If we now create a document, we can see the log property stored in the document because it was injected by RESTHeart in the request data.

$ http -a a:a PUT a:=1
HTTP/1.1 201 Created
$ http -a a:a GET
HTTP/1.1 200 OK
    "_id": "mydoc", 
    "a": 1, 
    "log": {
        "remoteIp": "", 
        "userName": "a"

Filter out properties with filterProperties transformer

filterProperties is a transformer shipped with RESTHeart that allows to filter out a the properties specified via the args property of the transformer metadata object.

The usual application of this transformer is hiding sensitive data to clients.

In Security section, the DbIdentityManager is described. It allows to authenticate users defined in a mongodb collection that must have the following fields: _id as the userid, password (string) and roles (an array of strings).

Let's say that the user collection is called userbase; we can remove the password to be sent back to clients with the following filterProperties transformer.

http -a a:a PUT rts:='[{"name":"filterProperties", "phase":"RESPONSE", "scope":"CHILDREN", "args":["password"]}]'

Custom Transformers

A transformer is a java class that implements the interface org.restheart.hal.metadata.singletons.Transformer.

It requires to implement the method tranform() with 4 arguments:

  1. HttpServerExchange exachange
  2. RequestContext context (that is the suggested way to retrieve the information of the request such as the payload) 
  3. DBObject contentToTransform (the json document to transform)
  4. DBObject args (that is the arguments passed via the args property of the transformer metadata object).


public interface Transformer {
    void tranform(final HttpServerExchange exchange, final RequestContext context, DBObject contentToTransform, final DBObject args);

Once a transformer has been implemented, it can be given a name (to be used as the name property of the transformer metadata object) in the configuration file.

The following is the default configuration file section declaring the off-the-shelf transformers provided with RESTHeart plus custom one.

- group: transformers
      interface: org.restheart.hal.metadata.singletons.Transformer
        - name: addRequestProperties
          class: org.restheart.hal.metadata.singletons.RequestPropsInjecterTransformer
        - name: filterProperties
          class: org.restheart.hal.metadata.singletons.FilterTransformer
        - name: stringsToOids
          class: org.restheart.hal.metadata.singletons.ValidOidsStringsAsOidsTransformer
        - name: oidsToStrings
          class: com.whatever.MyTransformer
        - name: myTransformer

Or course, the class of the custom transformer must be added to the java classpath.

For example, RESTHeart could be started with the following command:


$ java -server -classpath restheart.jar:custom-transformer.jar org.restheart.Bootstrapper restheart.yml

The following code, is an example transformer that adds to the content the property timestamp.

import org.restheart.hal.metadata.singletons.Transformer;
import io.undertow.server.HttpServerExchange;
import org.restheart.handlers.RequestContext;
import com.mongodb.DBObject;
package com.whatever;
public class MyTransformer implements Transformer {
    tranform(final HttpServerExchange exchange, final RequestContext context, DBObject contentToTransform, final DBObject args) {
        contentToTransform.put("_timestamp", System.currentTimeMillis());
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