This documentation describes the GORM API mechanics and how a datastore implementation can be built to interface to any database providing a GORM API onto it. This documentation is mainly targeted at developers interested in creating implementations of GORM ontop of alternative datastores.

As of this writing the project has several implementations of GORM against a variety of different datastore implementations. Current implementations include:

  • Hibernate 3,4 and 5

  • MongoDB

  • Redis

  • Neo4j

  • Cassandra

  • java.util.ConcurrentHashMap (the fastest datastore in the world)

The remainder of this document describes how the project is structured, how to build a project and how to implement a GORM provider.

Getting Started

Checking out and Building

The project is currently hosted on Github at [].

You are free to fork the project from there or clone it anonymously using git:

git clone
cd grails-data-mapping

The project has a Gradle build. You will need Intellij 15 or greater to work with the source code.

Use the Intellij 15 Gradle tooling to import the project.

To build the project you can run the assemble task:

./gradlew assemble

To install the jar files for the various subprojects into your local Maven repository you can run:

./gradlew install

To build all of the documentation run the command:

./gradlew allDocs

Documentation will produced in the build/docs directory.

If you experience PermGen errors when building documentation you may need to increase the JVM permgen inside GRADLE_OPTS

Project Structure

The project is essentially a multi-project Gradle build. There is a core API and then subprojects that implement that API. The core API subprojects include:

  • grails-datastore-core - The core API, this provides core interfaces for implementing a GORM provider

  • grails-datastore-gorm - The runtime meta-programming and AST transformation infrastructure behind GORM. Also provides end users APIs like grails.gorm.CriteriaBuilder and grails.gorm.DetachedCriteria

  • grails-datastore-gorm-support - Support classes for easing the writing of a GORM plugin for Grails

  • grails-datastore-gorm-tck - The TCK that includes hundreds of Spock specifications that a GORM implementation will need to pass

  • grails-datastore-web - Classes required to integrate GORM into a web tier

Beyond these core subprojects there are implementations for various datastores. For example:

The documentation for each implementation is kept in the documentation subprojects that start with grails-documentation. There are documentation projects for the core API, MongoDB, Neo4j, Redis, and Cassandra.

Finally the Grails 3 plugins that are used to distribute the GORM implementations to end users can be found in the grails-plugins directory and the grails2-plugins directory for Grails 2.x.

Understanding the GORM API


The GORM Developer API is split into a low-level API that implementors need to implement for each individual datastore and then set of higher level APIs that enhance domain classes with things regular users see such as dynamic finders, criteria queries and so on.

The low-level API classes are found in the grails-datastore-core subproject, whilst the higher level APIs used to enhance domain classes are found in grails-datastore-gorm. In this section we will discuss the low-level API.

Datastore Basics

The MappingContext

The org.grails.datastore.mapping.model.MappingContext interface is used to obtain metadata about the classes that are configured for persistence. There are org.grails.datastore.mapping.model.PersistentEntity and org.grails.datastore.mapping.model.PersistentProperty interfaces that represent a class and its properties respectively. These can be obtained and introspected via the MappingContext.

There are various concrete implementations of the MappingContext interface such as:

  • DocumentMappingContext - Used for document stores, subclassed by MongoMappingContext

  • JpaMappingContext - Used for JPA

  • KeyValueMappingContext - Used by key/value stores

Creating a new MappingContext may be useful because it allows users to configure how a class is mapped to the underlying datastore using GORM’s mapping block as well as allowing registration of custom type converters and so on. The implementation for Neo4j looks like this:

class Neo4jMappingContext extends AbstractMappingContext {

    MappingFactory<Collection, Attribute> mappingFactory
    MappingConfigurationStrategy syntaxStrategy

    Neo4jMappingContext() {
        mappingFactory = new GraphGormMappingFactory()
        syntaxStrategy = new GormMappingConfigurationStrategy(mappingFactory)
        //addTypeConverter(new StringToNumberConverterFactory().getConverter(BigDecimal))
        addTypeConverter(new StringToShortConverter())
        addTypeConverter(new StringToBigIntegerConverter())

    protected PersistentEntity createPersistentEntity(Class javaClass) {
        GraphPersistentEntity persistentEntity = new GraphPersistentEntity(javaClass, this)
        mappingFactory.createMappedForm(persistentEntity) // populates mappingFactory.entityToPropertyMap as a side effect

    MappingConfigurationStrategy getMappingSyntaxStrategy() {

    MappingFactory getMappingFactory() {

Notice how Neo4j provides a custom GraphGormMappingFactory and GraphPersistentEntity to allow the domain class configuration to be changed for a given Neo4j Node.

The Datastore Interface

The org.grails.datastore.mapping.core.Datastore interface is the equivalent of a SQL DataSource where by it provides the necessary capability to create a connection. In most cases one can simply subclass the AbstractDatastore super class and implement the createSession method. The following implementation is from the SimpleMapDatastore which implements GORM ontop of a ConcurrentHashMap:

protected Session createSession(PropertyResolver connDetails) {
    return new SimpleMapSession(this, getMappingContext(), getApplicationEventPublisher());

The implementation depends a lot on the underlying datastore. For example for MongoDB the following implementation is used:

protected Session createSession(PropertyResolver connDetails) {
    return new MongoSession(this, getMappingContext(), getApplicationEventPublisher(), false);

Notice that the Datastore also has a reference to the MappingContext discussed in the previous section.

The Session Interface

The org.grails.datastore.mapping.core.Session interface represents an active connection. It can be either stateful or stateless, depending on the implementation. For example of embedded databases where there is no network connection, a stateful session is not particularly useful, but a datastore that creates network connections you may want to cache returned instances to reduce load.

The AbstractSession class provides some support for creating stateful sessions, if you prefer a stateless implementation then simply implement Session or subclass AbstractAttributeStoringSession.

In general if you subclass AbstractSession the minimum you need to do is implement the createPersister method:

protected Persister createPersister(Class cls, MappingContext mappingContext) {
    PersistentEntity entity = mappingContext.getPersistentEntity(cls.getName());
    if (entity == null) {
        return null;
    return new SimpleMapEntityPersister(mappingContext, entity, this,
        (SimpleMapDatastore) getDatastore(), publisher);

The example above is from the SimpleMapSession implementation, which creates a SimpleMapEntityPersister instance and returns it. Returning null indicates that the class cannot be persisted and an exception will be thrown

Implementing CRUD

The EntityPersister Interface

The EntityPersister interface is used to implement the basic Create, Read, Update and Delete (CRUD) operations. There are individual methods to implement such as persistEntity, updateEntity, deleteEntity and so on.

In many cases there is a representation of an entity in its "native" form as supplied by the datastore driver. For example in Cassandra this could be a ColumnFamily, or in MongoDB a DBCollection.

To support implementation such cases there is an abstract NativeEntryEntityPersister<T, K> super class that provides the basis for an implementation that maps a native entry, such as a MongoDB DBObject or a Neo4j Node to a persist entity and back again.

The 2 generic types of this superclass indicate the native entry type (example DBObject in MongoDB) and the native key type (example ObjectId in MongoDB). The MongoDB implementation looks like this:

public class MongoEntityPersister extends NativeEntryEntityPersister<DBObject, Object>

Note that Object is used for the key since MongoDB also supports Long and String-based identifiers.

They key methods that need implementing are defined below:

  • getEntityFamily() - Defines the the name of the entity group or family. This could be a database table, a Cassandra Column Family or a MongoDB collection.

  • T createNewEntry(String family) - Creates a native entry ready to be inserted

  • Object getEntryValue(T nativeEntry, String property) - Retrieves a value of entry and returns its Java object form. For example a "date" property stored as a String in the datastore would need to b returned as a java.util.Date at this point

  • setEntryValue(T nativeEntry, String key, Object value) - Sets a value of the native entry, converting any Java objects to the required native format

  • deleteEntry(String family, K key, Object entry) - Deletes an entry for the given family, native key and entry

  • T retrieveEntry(PersistentEntity persistentEntity, String family, Serializable key) - Retrieves a native entry for the given entity, family and key

  • K storeEntry(PersistentEntity persistentEntity, EntityAccess entityAccess, K storeId, T nativeEntry) - Stores a native entry for the given id

  • updateEntry(PersistentEntity persistentEntity, EntityAccess entityAccess, K key, T entry) - Updates an entry

  • K generateIdentifier(PersistentEntity persistentEntity, T entry) - Generate an identifier for the given native entry

  • PropertyValueIndexer getPropertyIndexer(PersistentProperty property) - If the datastore requires manual indexing you’ll need to implement a PropertyIndexer otherwise return null

  • AssociationIndexer getAssociationIndexer(T nativeEntry, Association association) - If the datastore requires manual indexing you’ll need to implement a AssociationIndexer otherwise return null


The createNewEntry method is used to create a native record that will be inserted into the datastore. In MongoDB this is a DBObject whilst in the implementation for ConcurrentHashMap it is another Map:

protected DBObject createNewEntry(String family) {
    return new BasicDBObject();


The retrieveEntry method is used to retrieve a native record for a given key:

protected DBObject retrieveEntry(final PersistentEntity persistentEntity,
        String family, final Serializable key) {
    return mongoTemplate.execute(new DbCallback<DBObject>() {
        public DBObject doInDB(DB con) throws MongoException, DataAccessException {
            DBCollection dbCollection = con.getCollection(getCollectionName(persistentEntity));
            return dbCollection.findOne(key);

Here you can see the MongoDB implementation that uses a Spring Data MongoTemplate to find a DBObject for the given key. There is a separate storeEntry method that is used to actually store the native object. In MongoDB this looks like:

protected Object storeEntry(final PersistentEntity persistentEntity, final EntityAccess entityAccess,
                            final Object storeId, final DBObject nativeEntry) {
    return mongoTemplate.execute(new DbCallback<Object>() {
        public Object doInDB(DB con) throws MongoException, DataAccessException {
            nativeEntry.put(MONGO_ID_FIELD, storeId);
            return storeId;

Notice it doesn’t actually do anything native insert into a MongoDB collection. This is because the Datastore API supports the notion of batch insert operations and flushing. In the case of MongoDB the MongoSession implementation overrides the flushPendingInserts method of AbstractSession and performs a batch insert of multiple MongoDB documents (ie `DBObject`s) at once:

collection.insert(dbObjects.toArray(new DBObject[dbObjects.size()]), writeConcernToUse);

Other datastores that do not support batch inserts would instead to the insert in the storeEntry method itself. For example the implementation for ConcurrentHashMap looks like (note Groovy code):

protected storeEntry(PersistentEntity persistentEntity, EntityAccess entityAccess, storeId, Map nativeEntry) {
    if (!persistentEntity.root) {
        nativeEntry.discriminator = persistentEntity.discriminator
    datastore<<family>>.put(storeId, nativeEntry)
    return storeId


The updateEntry method is used to update an entry:

public void updateEntry(final PersistentEntity persistentEntity, final EntityAccess ea,
        final Object key, final DBObject entry) {
    mongoTemplate.execute(new DbCallback<Object>() {
        public Object doInDB(DB con) throws MongoException, DataAccessException {
            String collectionName = getCollectionName(persistentEntity, entry);
            DBCollection dbCollection = con.getCollection(collectionName);
            if (isVersioned(ea)) {
                // TODO this should be done with a CAS approach if possible
                DBObject previous = dbCollection.findOne(key);
                checkVersion(ea, previous, persistentEntity, key);

            MongoSession mongoSession = (MongoSession) session;
            dbCollection.update(dbo, entry, false, false, mongoSession.getWriteConcern());
            return null;

As you can see again the underlying database specific update method is used, in this case the DBCollection’s `update method.


The deleteEntry method is used to delete an entry. For example in the ConcurrentHashMap implementation it is simply removed from the map:

protected void deleteEntry(String family, key, entry) {

Whilst in MongoDB the DBCollection object’s remove method is called:

protected void deleteEntry(String family, final Object key, final Object entry) {
    mongoTemplate.execute(new DbCallback<Object>() {
        public Object doInDB(DB con) throws MongoException, DataAccessException {
            DBCollection dbCollection = getCollection(con);

            MongoSession mongoSession = (MongoSession) session;
            dbCollection.remove(key, mongoSession.getWriteConcern());
            return null;

        protected DBCollection getCollection(DB con) {
            return con.getCollection(getCollectionName(getPersistentEntity()));

Note that if the underlying datastore supports batch delete operations you may want override and implement the deleteEntries method which allows for deleting multiple entries in a single operation. The implementation for MongoDB looks like:

protected void deleteEntries(String family, final List<Object> keys) {
    mongoTemplate.execute(new DbCallback<Object>() {
        public Object doInDB(DB con) throws MongoException, DataAccessException {
            String collectionName = getCollectionName(getPersistentEntity());
            DBCollection dbCollection = con.getCollection(collectionName);

            MongoSession mongoSession = (MongoSession) getSession();
            MongoQuery query = mongoSession.createQuery(getPersistentEntity().getJavaClass());
  , keys);


            return null;

You’ll notice this implementation uses a MongoQuery instance. Note that implementing an EntityPersister you have enabled basic CRUD operations, but not querying, which is a topic of the following sections. First, however secondary indices need to covered since they are required for querying.

Secondary Indexing

Unresolved directive in index.adoc - include::understandingApi/secondaryIndexes.adoc[]

Implementing Querying


The org.grails.datastore.mapping.query.Query abstract class defines the query model and it is the job of the GORM implementor to translate this query model into an underlying database query. This is different depending on the implementation and may involve:

  • Generating a String-based query such as SQL or JPA-QL

  • Creating a query object such as MongoDB’s use of a Document to define queries

  • Generating for use with manually created Secondary indices as is the case with Redis

The Query object defines the following:

  • One or many Criterion that define the criteria to query by.

  • Zero or many Projection instances that define what the data you want back will look like.

  • Pagination parameters such as max, offset

  • Sorting parameters

There are many types of Criterion for each specific type of query, examples include Equals, Between, Like etc. Depending on the capabilities of the underlying datastore you may implement only a few of these.

There are also many types of Projection such as SumProjection, MaxProjection and CountProjection. Again you may implement only a few of these.

If the underlying datastore doesn’t for example support calculating a sum or max of a particular property, there is a ManualProjections class that you can use to perform these operations in memory on the client.

Writing a Query implementation is probably the most complex part of implementing a GORM provider, but starts by subclassing the Query class and implementing the executeQuery method:

public class MongoQuery extends Query implements QueryArgumentsAware {


Using the Query Model

To implement querying you need to understand the Query model. As discussed a Query contains a list of Criterion, however the root Criterion could be a conjunction (an AND query) or a disjunction (an OR query). The Query may also contain a combination of regular criterion (=, !=, LIKE etc.) and junctions (AND, OR or NOT). Implementing a Query therefore requires writing a recursive method. The implementation for ConcurrentHashMap looks like

Collection executeSubQueryInternal(criteria, criteriaList) {
    SimpleMapResultList resultList = new SimpleMapResultList(this)
    for (Query.Criterion criterion in criteriaList) {
        if (criterion instanceof Query.Junction) {
            resultList.results << executeSubQueryInternal(criterion, criterion.criteria)
        else {
            PersistentProperty property = getValidProperty(criterion)
            def handler = handlers[criterion.getClass()]

            def results = handler?.call(criterion, property) ?: []
            resultList.results << results

Notice that if a Junction is encountered (which represents an AND, OR or NOT) then the method recurses to handle the junctions, otherwise a handler for the Criterion class is obtained and executed. The handlers map is a map of Criterion class to query handlers. The implementation for Equals looks like:

def handlers = [
    (Query.Equals): { Query.Equals equals, PersistentProperty property->
        def indexer = entityPersister.getPropertyIndexer(property)
        final value = subqueryIfNecessary(equals)
        return indexer.query(value)

Which simply uses the property indexer to query for all identifiers. Of course here we are a describing a case of a datastore (in this case ConcurrentHashMap) which doesn’t support secondary indices. It may be that instead of manually querying the secondary indices in this way that you simply build a String-based or native query. For example in MongoDB this looks like:

queryHandlers.put(Equals.class, new QueryHandler<Equals>() {
    public void handle(PersistentEntity entity, Equals criterion, Document query) {
        String propertyName = getPropertyName(entity, criterion);
        Object value = criterion.getValue();
        PersistentProperty property = entity.getPropertyByName(criterion.getProperty());
        MongoEntityPersister.setDBObjectValue(query, propertyName, value, entity.getMappingContext());

Notice how the query in this case is a DBObject. For Gemfire again the implementation is different:

queryHandlers.put(Equals.class, new QueryHandler() {
    public int handle(PersistentEntity entity, Criterion criterion, StringBuilder q, List params, int index) {
        Equals eq = (Equals) criterion;
        final String name = eq.getProperty();
        validateProperty(entity, name, Equals.class);

        q.append(calculateName(entity, name));
        return appendOrEmbedValue(q, params, index, eq.getValue(), EQUALS);

In this case a StringBuilder is used to construct a OQL query from the Query model.

GORM Enhancer

Once you have implemented the lower-level APIs you can trivially provide a GORM API to a set of Grails domain classes. For example consider the following simple domain class:

import grails.persistence.*

class Book {
    String title

The following setup code can be written to enable GORM for MongoDB:

// create context
def context = new MongoMappingContext(databaseName)

// create datastore
def mongoDatastore = new MongoDatastore(context)

// enhance
def enhancer = new MongoGormEnhancer(mongoDatastore, new DatastoreTransactionManager(datastore: mongoDatastore))

// use GORM!
def books = Book.list()

They key part to enabling the usage of all the GORM methods (list(), dynamic finders etc.) is the usage of the MongoGormEnhancer. This class subclasses org.grails.datastore.gorm.GormEnhancer and provides some extensions to GORM specific to MongoDB. A subclass is not required however and if you don’t require any datastore specific extensions you can just as easily use the regular GormEnhancer:

def enhancer = new GormEnhancer(mongoDatastore, new DatastoreTransactionManager(datastore: mongoDatastore))

Adding to GORM APIs

By default the GORM compiler will make all GORM entities implement the GormEntity trait. Which provide all of the default GORM methods. However if you want to extend GORM to provide more methods specific to a given data store you can do so by extending this trait.

For example Neo4j adds methods for Cypher querying:

trait Neo4jEntity<D> extends GormEntity<D> {

    static Result cypherStatic(String queryString, Map params ) {
        def session = AbstractDatastore.retrieveSession(Neo4jDatastore)
        def graphDatabaseService = (GraphDatabaseService)session.nativeInterface
        graphDatabaseService.execute(queryString, params)

With this addition then you then need to tell the GORM compiler to make entities implement this trait. To do that implement a TraitProvider:

package org.grails.datastore.gorm.neo4j

import grails.neo4j.Neo4jEntity
import groovy.transform.CompileStatic
import org.grails.compiler.gorm.GormEntityTraitProvider

class Neo4jEntityTraitProvider implements GormEntityTraitProvider {
    final Class entityTrait = Neo4jEntity

And then add a src/main/resources/META-INF/services/org.grails.compiler.gorm.GormEntityTraitProvider file specifying the name of your trait provider:


GORM will automatically inject to trait into any domain class found in grails-app/domain or annotated with the Entity annotation, unless Hibernate is on the classpath in which case you have to tell GORM to map the domain class with Neo4j:

static mapWith = "neo4j"

Using the Test Compatibility Kit

The grails-datastore-gorm-tck project provides a few hundred tests that ensure a particular GORM implementation is compliant. To use the TCK you need to define a dependency on the TCK in the subprojects build.gradle file:

testCompile project(':grails-datastore-gorm-tck')

Then create a Setup.groovy file that sets up your custom datastore in your implementation.

For example the ConcurrentHashMap implementation has one defined in grails-datastore-gorm-test/src/test/groovy/org/grails/datastore/gorm/Setup.groovy:

class Setup {

    static destroy() {
        // noop
    static Session setup(classes) {

        def ctx = new GenericApplicationContext()
        def simple = new SimpleMapDatastore(ctx)

        for (cls in classes) {

        def enhancer = new GormEnhancer(simple, new DatastoreTransactionManager(datastore: simple))

        simple.mappingContext.addMappingContextListener({ e -> enhancer.enhance e } as MappingContext.Listener)

        simple.applicationContext.addApplicationListener new DomainEventListener(simple)
        simple.applicationContext.addApplicationListener new AutoTimestampEventListener(simple)

        return simple.connect()

Some setup code has been omitted for clarity but basically the Setup.groovy class should initiase the Datastore and return a Session from the static setup method which gets passed a list of classes that need to be configured.

With this done all of the TCK tests will run against the subproject. If a particular test cannot be implemented because the underlying datastore doesn’t support the feature then you can create a test that matches the name of the test that is failing and it will override said test.

For example SimpleDB doesn’t support pagination so there is a grails.gorm.tests.PagedResultSpec class that overrides the one from the TCK. Each test is a Spock specification and Spock has an Ignore annotation that can be used to ignore a particular test:

 * Ignored for SimpleDB because SimpleDB doesn't support pagination
class PagedResultSpec extends GormDatastoreSpec{

Step by Step Guide to Creating and Implementation

To get started with your a new GORM implementation the following steps are required:

Initial Directory Creation

$ git clone
$ cd grails-data-mapping
$ mkdir grails-datastore-gorm-xyz

Setup Gradle Build

Create build.gradle:

$ vi grails-datastore-gorm-xyz/build.gradle

With contents:

dependencies {
    compile project(':grails-datastore-gorm'),

    testCompile project(':grails-datastore-gorm-tck')
    testRuntime "javax.servlet:javax.servlet-api:$servletApiVersion"


Add new project to settings.gradle in root project:

$ vi settings.gradle

Changes shown below:

// GORM Implementations

Create Project Source Directories

$ mkdir grails-datastore-gorm-xyz/src/main/groovy
$ mkdir grails-datastore-gorm-xyz/src/test/groovy

Generate IDE Project Files and Import into IDE

$ gradlew grails-datastore-gorm-xyz:idea


$ gradlew grails-datastore-gorm-xyz:eclipse

Implement Required Interfaces

In src/main/groovy create implementations:

  • extends and implements org.grails.datastore.mapping.core.AbstractDatastore

  • extends and implements org.grails.datastore.mapping.core.AbstractSession

  • extends and implements org.grails.datastore.mapping.engine.NativeEntryEntityPersister

  • extends and implements org.grails.datastore.mapping.query.Query

Create Test Suite

In src/test/groovy create org.grails.datastore.gorm.Setup class to configure TCK:

class Setup {

    static xyz
    static destroy() {
    static Session setup(classes) {
        def ctx = new GenericApplicationContext()
        xyz = new XyzDatastore(ctx)
        for (cls in classes) {

        def enhancer = new GormEnhancer(xyz, new DatastoreTransactionManager(datastore: xyz))

        xyz.mappingContext.addMappingContextListener({ e -> enhancer.enhance e } as MappingContext.Listener)
        xyz.applicationContext.addApplicationListener new DomainEventListener(xyz)
        xyz.applicationContext.addApplicationListener new AutoTimestampEventListener(xyz)


Then in src/test/groovy create test suite class to allow running tests in IDE (without this you won’t be able to run TCK tests from the IDE). Example test suite:

package org.grails.datastore.gorm

import org.junit.runners.Suite.SuiteClasses
import org.junit.runners.Suite
import org.junit.runner.RunWith
import grails.gorm.tests.*

 * @author graemerocher
class XyzTestSuite {

Implement the TCK!

Keep iterating until you have implemented all the tests in the TCK.