(Quick Reference) # 20 Grails and Hibernate Version: 3.2.8 ## Table of Contents # 20 Grails and Hibernate If GORM (Grails Object Relational Mapping) is not flexible enough for your liking you can alternatively map your domain classes using Hibernate, either with XML mapping files or JPA annotations. You will be able to map Grails domain classes onto a wider range of legacy systems and have more flexibility in the creation of your database schema. Best of all, you will still be able to call all of the dynamic persistent and query methods provided by GORM! ## 20.1 Using Hibernate XML Mapping Files Mapping your domain classes with XML is pretty straightforward. Simply create a `hibernate.cfg.xml` file in your project’s `grails-app/conf` directory, either manually or with the create-hibernate-cfg-xml command, that contains the following:
    <?xml version='1.0' encoding='UTF-8'?>
    <!DOCTYPE hibernate-configuration PUBLIC
            "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
            "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
    <hibernate-configuration>
        <session-factory>
            <!-- Example mapping file inclusion -->
            <mapping resource="org.example.Book.hbm.xml"/>
            ...
        </session-factory>
    </hibernate-configuration>
The individual mapping files, like 'org.example.Book.hbm.xml' in the above example, also go into the `grails-app/conf` directory. To find out how to map domain classes with XML, check out the Hibernate manual. If the default location of the `hibernate.cfg.xml` file doesn’t suit you, you can change it by specifying an alternative location in `grails-app/conf/application.groovy`:
    hibernate {
        config.location = "file:/path/to/my/hibernate.cfg.xml"
    }
or even a list of locations:
    hibernate {
        config.location = ["file:/path/to/one/hibernate.cfg.xml",
                           "file:/path/to/two/hibernate.cfg.xml"]
    }
Grails also lets you write your domain model in Java or reuse an existing one that already has Hibernate mapping files. Simply place the mapping files into `grails-app/conf` and either put the Java files in `src/main/groovy/` or the classes in the project’s `lib` directory if the domain model is packaged as a JAR. You still need the `hibernate.cfg.xml` though! ## 20.2 Mapping with Hibernate Annotations To map a domain class with annotations, create a new class in `src/main/groovy/` and use the annotations defined as part of the EJB 3.0 spec (for more info on this see the Hibernate Annotations Docs):
    package com.books;

    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;

    @Entity
    public class Book {
        private Long id;
        private String title;
        private String description;
        private Date date;

        @Id
        @GeneratedValue
        public Long getId() {
            return id;
        }

        public void setId(Long id) {
            this.id = id;
        }

        public String getTitle() {
            return title;
        }

        public void setTitle(String title) {
            this.title = title;
        }

        public String getDescription() {
            return description;
        }

        public void setDescription(String description) {
            this.description = description;
        }
    }
Then register the class with the Hibernate `sessionFactory` by adding relevant entries to the `grails-app/conf/hibernate.cfg.xml` file as follows:
    <!DOCTYPE hibernate-configuration SYSTEM
      "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
    <hibernate-configuration>
        <session-factory>
            <mapping package="com.books" />
            <mapping class="com.books.Book" />
        </session-factory>
    </hibernate-configuration>
See the previous section for more information on the `hibernate.cfg.xml` file. When Grails loads it will register the necessary dynamic methods with the class. To see what else you can do with a Hibernate domain class see the section on scaffolding. ## 20.3 Adding Constraints You can still use GORM validation even if you use a Java domain model. Grails lets you define constraints through separate scripts in the `src/main/groovy/` directory. The script must be in a directory that matches the package of the corresponding domain class and its name must have a Constraints suffix. For example, if you had a domain class `org.example.Book`, then you would create the script `src/main/groovy/org/example/BookConstraints.groovy`\. Add a standard GORM `constraints` block to the script:
    constraints = {
        title blank: false
        author blank: false
    }
Once this is in place you can validate instances of your domain class! | Quick Reference (hide)