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Groovy Mixins?

Answer 1

+1 A:

I guess what you've seen there is rather a proposal than a feature ;) Groovy does not support mixins out of the box in this way yet (if ever). But there is a 3rd party lib that can be used to emulate such a behavour: Injecto. And mixins can be defined using AST-Macros in the 1.6 version of Groovy (which is not final yet).

You should always check if your're reading the docs from the real groovy project or from the GroovyJSR project (which is rather a place where proposals are collected).

Another way is to use plain-old MOP to inject behaviour into groovy classes by modifying metaClasses.

Cheers

Siegfried Puchbauer 2009-01-08 20:16:36

Answer 2

A:

FYI: There is such a thing as "embedded" domains in Grails now, but it has problems. This is where you can logically include one domain as part of another and have its fields all occur physically in the one DB table. For example, if you find that you have the same subset of fields appearing in multiple tables, like street address/city/state/zip, you could define a StreetAddress domain and embed it. One of the current problems is that Grails will still create a street_address table in addition to embedding its fields in the other tables (unless you play tricks). There are submitted patches pending for this, it seems.

2009-01-21 05:26:25

Answer 3

+5 A:

I don't think you are using the proper mixin syntax. Try this:

class MyMixin {
    static doStuff(Person) {
        'stuff was done'
    }
}

class Person {}

Person.mixin MyMixin

new Person().doStuff()

Matthew Taylor 2009-01-22 15:07:19

Answer 4

+2 A:

Since Groovy 1.6 you can either apply a mixin at compile-time to a class using an annotation

@Mixin(ImagesMixin)
class Person {
}

Or you can apply the mixin at runtime like this:

def myMixin = ImagesMixin
Person.mixin myMixin

The latter approach is more dynamic as the class to mixin can be determined at runtime. Further information about Groovy mixins is available here.

In my experience, a lot of meta-programming of domain classes simply doesn't work. I don't exactly know why, but suspect it's due to the fact these classes are already very heavily meta-programmed by the Grails runtime. In general my approach is

Try the meta-programming on a POGO in the Groovy console
If that works, try it on a non-domain class in the Grails console
If that works, try it on a domain class in the Grails console. If it doesn't work, then it must be because it's a domain class (rather than a problem with the syntax). At this point it's advisable to try and find another way of accomplishing your goal. If that's not possible, then use a combination of the Grails mailing list and/or Stackoverflow and/or the Grails source code to try and get the meta-programming working.

Don 2009-08-13 20:42:51

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