将JSON绑定到嵌套的Grails域对象

Question

I'm developing a RESTful interface which is used to provide JSON data for a JavaScript application.

On the server side I use Grails 1.3.7 and use GORM Domain Objects for persistence. I implemented a custom JSON Marshaller to support marshalling the nested domain objects

Here are sample domain objects:

class SampleDomain {
    static mapping = { nest2 cascade: 'all' }
    String someString
    SampleDomainNested nest2
}

and

class SampleDomainNested {
    String someField
}

The SampleDomain resource is published under the URL /rs/sample/ so /rs/sample/1 points to the SampleDomain object with ID 1

When I render the resource using my custom json marshaller (GET on /rs/sample/1), I get the following data:

{
    "someString" : "somevalue1",
    "nest2" : {
        "someField" : "someothervalue"
    }
}

which is exactly what I want.

Now comes the problem: I try to send the same data to the resource /rs/sample/1 via PUT.

To bind the json data to the Domain Object, the controller handling the request calls def domain = SampleDomain.get(id) and domain.properties = data where data is the unmarshalled object.

The binding for the "someString" field is working just fine, but the nested object is not populated using the nested data so I get an error that the property "nest2" is null, which is not allowed.

I already tried implementing a custom PropertyEditorSupport as well as a StructuredPropertyEditor and register the editor for the class.

Strangely, the editor only gets called when I supply non-nested values. So when I send the following to the server via PUT (which doesn't make any sense ;) )

{
    "someString" : "somevalue1",
    "nest2" : "test"
}

at least the property editor gets called.

I looked at the code of the GrailsDataBinder . I found out that setting properties of an association seems to work by specifying the path of the association instead of providing a map, so the following works as well:

{
    "someString" : "somevalue1",
    "nest2.somefield" : "someothervalue"
}

but this doesn't help me since I don't want to implement a custom JavaScript to JSON object serializer.

Is it possible to use Grails data binding using nested maps? Or do I really heave to implement that by hand for each domain class?

Thanks a lot,

Martin

Answer 1

Since this question got upvoted several times I would like to share what I did in the end:

Since I had some more requirements to be implemented like security etc. I implemented a service layer which hides the domain objects from the controllers. I introduced a "dynamic DTO layer" which translates Domain Objects to Groovy Maps which can be serialized easily using the standard serializers and which implements the updates manually. All the semi-automatic/meta-programming/command pattern/... based solutions I tried to implement failed at some point, mostly resulting in strange GORM errors or a lot of configuration code (and a lot of frustration). The update and serialization methods for the DTOs are fairly straightforward and could be implemented very quickly. It does not introduce a lot of duplicate code as well since you have to specify how your domain objects are serialized anyway if you don't want to publish your internal domain object structure. Maybe it's not the most elegant solution but it was the only solution which really worked for me. It also allows me to implement batch updates since the update logic is not connected to the http requests any more.

However I must say that I don't think that grails is the appropriate tech stack best suited for this kind of application, since it makes your application very heavy-weight and inflexbile. My experience is that once you start doing things which are not supported by the framework by default, it starts getting messy. Furthermore, I don't like the fact that the "repository" layer in grails essentially only exists as a part of the domain objects which introduced a lot of problems and resulted in several "proxy services" emulating a repository layer. If you start building an application using a json rest interface, I would suggest to either go for a very light-weight technology like node.js or, if you want to/have to stick to a java based stack, use standard spring framework + spring mvc + spring data with a nice and clean dto layer (this is what I've migrated to and it works like a charm). You don't have to write a lot of boilerplate code and you are completely in control of what's actually happening. Furthermore you get strong typing which increases developer productivity as well as maintainability and which legitimates the additional LOCs. And of course strong typing means strong tooling!

I started writing a blog entry describing the architecture I came up with (with a sample project of course), however I don't have a lot of time right now to finish it. When it's done I'm going to link to it here for reference.

Hope this can serve as inspiration for people experiencing similar problems.

Cheers!

Answer 2

It requires you to provide teh class name:

{ class:"SampleDomain", someString: "abc", 
nest2: { class: "SampleDomainNested", someField:"def" }
} 

I know, it requires different input that the output it produces.

As I mentioned in the comment earlier, you might be better off using the gson library.

Answer 3

Not sure why you wrote your own json marshaller, with xstream around.

See http://x-stream.github.io/json-tutorial.html

We have been very happy with xstream for our back end (grails based) services and this way you can render marshall in xml or json, or override the default marshalling for a specific object if you like.

Jettison seems to produce a more compact less human readable JSON and you can run into some library collision stuff, but the default internal json stream renderer is decent.

If you are going to publish the service to the public, you will want to take the time to return appropriate HTTP protocol responses for errors etc... ($.02)