Many-to-many relationship mapping with GORM/Grails

Author: Chris Broadfoot
Posted on 19 July, 2008

I’ve been working on a pet project in my spare time – a sort of twitter clone that authenticates against Atlassian Crowd. I’m using the awesome Grails/Crowd plugin, which is pretty seamless (but needs a bit of internal polishing).

I’ve only scratched the surface of Grails, but here are a few things I’ve run into as a Grails newb.

Many-to-many relationships and cascading updates

GORM (Grails’s abstraction of Hibernate) lets you define a many-to-many relationship like this:

class User {
    String name
    static hasMany = [groups: Group]
}

class Group {
    String name
    static hasMany = [members: User]
    static belongsTo = User
}

This relationship can be accessed through some helpful dynamic methods that Grails provides:

new User(name: "foo").addToGroups(name: "bar").save()
// creates a new user AND group, saves both of them 
// and the relationship 

def user = new User(name: "foo").save()
new Group(name: "bar").save().addToMembers(user)
// as above, but addToMembers will not cascade the relationship 
// to the user

The crucial part here is the belongsTo property of the domain classes. This tells GORM which class should ‘own’ the relationship. What this means is that if you want the relationship to cascade through both objects, you will need to call the addTo* method on the owning object.

Many-to-many relationships with the same class

Back to the pet project – one of the features I really wanted was the ability to subscribe to status updates for a whole group.

Here’s a quick ERD (generated by the excellent IDEA/Grails integration) of the domain classes:

You can quickly see that there are two many-to-many relationships between the classes User and Group.

The code looks something like this now (other stuff stripped out):

class User {
    String name
    static hasMany = [groups: Group, groupSubscriptions: Group]
    static mappedBy = [groups: "members"]
}

class Group {
    String name
    static hasMany = [members: User]
    static belongsTo = User
    static mapping = {
        // we can't have a table called group
        table "group_"
    }
}

I thought that Grails would be smart enough to see what I was doing, but I was wrong (for various reasons). To cut to the chase, here’s the necessary code to implement this:

class User {
    String name
    static hasMany = [groups: Group, groupSubscriptions: Group]
    static mappedBy = [groups: "members",
                       groupSubscriptions: "subscribers"]

    static mapping = {
        groupSubscriptions joinTable: "user_group_s"
    }
}

class Group {
    String name
    static hasMany = [members: User, subscribers: User]
    static belongsTo = User

    static mapping = {
        // we can't have a table called group
        table "group_"
        subscribers joinTable: "user_group_s"
    }
}

Grails will create two relationship tables user_group_ and user_group_s. Having the back reference from Group to User isn’t so bad, since the collection is lazy loaded. I would have just preferred not to pollute the domain class.

Conclusion

I hope that someone out there finds this information useful, I’d hate to see another Grails newb fall into this big dark pit that I call lack of documentation.

That said, my experience with Grails and Groovy so far has been pretty awesome. If you’re looking to develop a web-app rapidly, I’d definitely be looking at Grails as an option.