7.2.1 Association in GORM 3.1.1

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(Quick Reference) # 7.2.1 Association in GORM - Reference Documentation Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith, Lari Hotari Version: 3.1.1 ## Table of Contents ## 7.2.1 Association in GORM Relationships define how domain classes interact with each other. Unless specified explicitly at both ends, a relationship exists only in the direction it is defined. ## 7.2.1.1 Many-to-one and one-to-one A many-to-one relationship is the simplest kind, and is defined with a property of the type of another domain class. Consider this example: ##### Example A
    class Face {
        Nose nose
    }
    class Nose {
    }
In this case we have a unidirectional many-to-one relationship from `Face` to `Nose`. To make this relationship bidirectional define the other side as follows (and see the section on controlling the ends of the association just below): ##### Example B
    class Face {
        Nose nose
    }
    class Nose {
        static belongsTo = [face:Face]
    }
In this case we use the `belongsTo` setting to say that `Nose` "belongs to" `Face`. The result of this is that we can create a `Face`, attach a `Nose` instance to it and when we save or delete the `Face` instance, GORM will save or delete the `Nose`. In other words, saves and deletes will cascade from `Face` to the associated `Nose`:
    new Face(nose:new Nose()).save()
The example above will save both face and nose. Note that the inverse is not true and will result in an error due to a transient `Face`:
    new Nose(face:new Face()).save() // will cause an error
Now if we delete the `Face` instance, the `Nose` will go too:
    def f = Face.get(1)
    f.delete() // both Face and Nose deleted
To make the relationship a true one-to-one, use the `hasOne` property on the owning side, e.g. `Face`: ##### Example C
    class Face {
        static hasOne = [nose:Nose]
    }
    class Nose {
        Face face
    }
Note that using this property puts the foreign key on the inverse table to the example A, so in this case the foreign key column is stored in the `nose` table inside a column called `face_id`. Also, `hasOne` only works with bidirectional relationships. Finally, it's a good idea to add a unique constraint on one side of the one-to-one relationship:

    class Face {
        static hasOne = [nose:Nose]    static constraints = {
            nose unique: true
        }
    }
    class Nose {
        Face face
    }
##### Controlling the ends of the association Occasionally you may find yourself with domain classes that have multiple properties of the same type. They may even be self-referential, i.e. the association property has the same type as the domain class it's in. Such situations can cause problems because Grails may guess incorrectly the type of the association. Consider this simple class:

    class Person {
        String name
        Person parent    static belongsTo = [ supervisor: Person ]    static constraints = { supervisor nullable: true }
    }
As far as Grails is concerned, the `parent` and `supervisor` properties are two directions of the same association. So when you set the `parent` property on a `Person` instance, Grails will automatically set the `supervisor` property on the other `Person` instance. This may be what you want, but if you look at the class, what we in fact have are two unidirectional relationships. To guide Grails to the correct mapping, you can tell it that a particular association is unidirectional through the `mappedBy` property:

    class Person {
        String name
        Person parent    static belongsTo = [ supervisor: Person ]    static mappedBy = [ supervisor: "none", parent: "none" ]    static constraints = { supervisor nullable: true }
    }
You can also replace "none" with any property name of the target class. And of course this works for normal domain classes too, not just self-referential ones. Nor is the `mappedBy` property limited to many-to-one and one-to-one associations: it also works for one-to-many and many-to-many associations as you'll see in the next section. If you have a property called "none" on your domain class, this approach won't work currently! The "none" property will be treated as the reverse direction of the association (or the "back reference"). Fortunately, "none" is not a common domain class property name. ## 7.2.1.2 One-to-many A one-to-many relationship is when one class, example `Author`, has many instances of another class, example `Book`. With Grails you define such a relationship with the `hasMany` setting:

    class Author {
        static hasMany = [books: Book]    String name
    }
    class Book {
        String title
    }
In this case we have a unidirectional one-to-many. Grails will, by default, map this kind of relationship with a join table. The ORM DSL allows mapping unidirectional relationships using a foreign key association instead Grails will automatically inject a property of type `java.util.Set` into the domain class based on the `hasMany` setting. This can be used to iterate over the collection:

    def a = Author.get(1)for (book in a.books) {
        println book.title
    }
The default fetch strategy used by Grails is "lazy", which means that the collection will be lazily initialized on first access. This can lead to the n+1 problem if you are not careful. If you need "eager" fetching you can use the ORM DSL or specify eager fetching as part of a query The default cascading behaviour is to cascade saves and updates, but not deletes unless a `belongsTo` is also specified:

    class Author {
        static hasMany = [books: Book]    String name
    }
    class Book {
        static belongsTo = [author: Author]
        String title
    }
If you have two properties of the same type on the many side of a one-to-many you have to use `mappedBy` to specify which the collection is mapped:
    class Airport {
        static hasMany = [flights: Flight]
        static mappedBy = [flights: "departureAirport"]
    }
    class Flight {
        Airport departureAirport
        Airport destinationAirport
    }
This is also true if you have multiple collections that map to different properties on the many side:
    class Airport {
        static hasMany = [outboundFlights: Flight, inboundFlights: Flight]
        static mappedBy = [outboundFlights: "departureAirport",
                           inboundFlights: "destinationAirport"]
    }
    class Flight {
        Airport departureAirport
        Airport destinationAirport
    }
## 7.2.1.3 Many-to-many Grails supports many-to-many relationships by defining a `hasMany` on both sides of the relationship and having a `belongsTo` on the owned side of the relationship:
    class Book {
        static belongsTo = Author
        static hasMany = [authors:Author]
        String title
    }
    class Author {
        static hasMany = [books:Book]
        String name
    }
Grails maps a many-to-many using a join table at the database level. The owning side of the relationship, in this case `Author`, takes responsibility for persisting the relationship and is the only side that can cascade saves across. For example this will work and cascade saves:
    new Author(name:"Stephen King")
            .addToBooks(new Book(title:"The Stand"))
            .addToBooks(new Book(title:"The Shining"))
            .save()
However this will only save the `Book` and not the authors!
    new Book(name:"Groovy in Action")
            .addToAuthors(new Author(name:"Dierk Koenig"))
            .addToAuthors(new Author(name:"Guillaume Laforge"))
            .save()
This is the expected behaviour as, just like Hibernate, only one side of a many-to-many can take responsibility for managing the relationship. Grails' Scaffolding feature does not currently support many-to-many relationship and hence you must write the code to manage the relationship yourself ## 7.2.1.4 Basic Collection Types As well as associations between different domain classes, GORM also supports mapping of basic collection types. For example, the following class creates a `nicknames` association that is a `Set` of `String` instances:
    class Person {
        static hasMany = [nicknames: String]
    }
GORM will map an association like the above using a join table. You can alter various aspects of how the join table is mapped using the `joinTable` argument:

    class Person {    static hasMany = [nicknames: String]    static mapping = {
           nicknames joinTable: [name: 'bunch_o_nicknames',
                               key: 'person_id',
                               column: 'nickname',
                               type: "text"]
        }
    }
The example above will map to a table that looks like the following: bunch_o_nicknames Table
    ---------------------------------------------
    | person_id         |     nickname          |
    ---------------------------------------------
    |   1               |      Fred             |
    ---------------------------------------------
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