I have several thoroughly unit-tested and finely crafted rich DDD model classes, with final immutable invariants and integrity checks. Object's instantiation happens through adequate constructors, static factory methods and even via Builders.
Now, I have to provide a Spring MVC form to create new instances of some classes.
It seems to me (I'm not an expert) that I have to provide empty constructor and attribute's setters for all form's backing classes I want to bind.
So, what should I do ?
Create anemic objects dedicated to form backing and transfer the informations to my domain model (so much for the DRY principle...) calling the appropriate methods / builder ?
Or is there a mecanisms that I missed that can save my day ? :)
Thank you in advance for your wisdom !
The objects that are used for binding with the presentation layers are normally called view models and they are DTOs purposed toward displaying data mapped from domain objects and then mapping user input back to domain objects. View models typically look very similar to the domain objects they represent however there are some important differences:
Data from the domain objects may be flattened or otherwise transformed to fit the requirements of a given view. Having the mapping be in plain objects is easier to manage than mappings in the presentation framework, such as MVC. It is easier to debug and detect errors.
A given view may require data from multiple domain objects - there may not be a single domain object that fits requirements of a view. A view model can be populated by multiple domain objects.
A view model is normally designed with a specific presentation framework in mind and as such may utilize framework specific attributes for binding and client side validation. As you stated, a typical requirement is for a parameterless constructor, which is fine for a view model. Again, it is much easier to test and manage a view model than some sort of complex mapping mechanism.
View models appear to violate the DRY principle, however after a closer look the responsibility of the view model is different, so with the single responsibility principle in mind it is appropriate to have two classes. Also, take a look at this article discussing the fallacy of reuse often lead by the DRY principle.
Furthermore, view models are indeed anemic, though they may have a constructor accepting a domain object as a parameter and a method for creating and updating a domain object using the values in the view model as input. From experience I find that it is a good practice to create a view model class for every domain entity that is going to be rendered by the presentation layer. It is easier to manage the double class hierarchy of domain objects and view models than it is to manage complex mapping mechanisms.
Note also, there are libraries that attempt to simplify the mapping between view models and domain objects, for example AutoMapper for the .NET Framework.
Yes you will need to create Objects for the form to take all the input, and the update the your model with this objects in one operation.
But I wont call this objects anemic (especially if you do DDD). This objects represent one unit of work. So this are Domain Concepts too!
I solved this by creating a DTO Interface:
public interface DTO<T> {
T getDomainObject();
void loadFromDomainObject(T domainObject);
}
public class PersonDTO implements DTO<Person> {
private String firstName;
private String lastName;
public PersonDTO() {
super();
}
// setters, getters ...
#Override
public Person getDomainObject() {
return new Person(firstName, lastName);
}
#Override
public void loadFromDomainObject(Person person) {
this.firstName = person.getFirstName();
this.lastName = person.getLastName();
}
// validation methods, view formatting methods, etc
}
This also stops view validation and formatting stuff from leaking into the domain model. I really dislike having Spring specific (or other framework specific) annotations (#Value, etc) and javax.validation annotations in my domain objects.
Related
so many pages and links i found, but none of them have clear answer. Even spring docs doesn't say anything about it.
So which one to use on what context?
Model is an interface that defines methods for adding/removing attributes.
From the javadoc:
Primarily designed for adding attributes to the model. Allows for
accessing the overall model as a java.util.Map.
ModelMap is just a Map implementation used for storing values that will be used in the UI.
ModelAndView is an object that combines a Model object (for storing values that will be used in the UI) and a View (used for resolving UIs components like jsp or templates). It was designed to provide both model and view in a single return.
I'm working on a project where we have multiple UI (view) models with DataAnnotations attributes for validation, mainly for input such as required fields, length, regular expressions, etc):
[Required]
public int Order { get; set; }
[Required]
[MaxLength(150)]
public string Title { get; set; }
The controller checks the ModelState to make sure objects are valid before handing them to the service layer.
Now, the service layer also validates the objects in order to accommodate for future changes that may require a new controller to support other output formats or another utility to import objects in batch.
My question is: Knowing that there should be a separation of concerns where controllers deal with UI models and Service layers deal with Business Objects, and therefore, I wouldn't like to pass UI model objects to the service layers, what's the preferred way to reuse the validation rules in the UI model objects? Or should they be replicated with code in the service layer? or should the DataAnnotations be added to the Business Objects too?
I'm using EF model first, so I don't see a simple way to add these annotations to the auto-generated POCOs.
Why not use FluentValidation? That way you can create a validator for each view model and run them in your UI layer and service layer if needs be. It also removes the need to decorate your view models with validation attributes. http://fluentvalidation.codeplex.com/
I just switched from ActiveRecord/NHibernate to Dapper. Previously, I had all of my queries in my controllers. However, some properties which were convenient to implement on my models (such as summaries/sums/totals/averages), I could calculate by iterating over instance variables (collections) in my model.
To be specific, my Project has a notion of AppSessions, and I can calculate the total number of sessions, plus the average session length, by iterating over someProject.AppSessions.
Now that I'm in Dapper, this seems confused: my controller methods now make queries to the database via Dapper (which seems okay), but my model class also makes queries to the database via Dapper (which seems strange).
TLDR: Should the DB access go in my model, or controller, or both? It seems that both is not correct, and I would like to limit it to one "layer" so that changing DB access style later doesn't impact too much.
You should consider using a repository pattern:
With repositories, all of the database queries are encapsulated within a repository which is exposed through public interface, for example:
public interface IGenericRepository<T> where T : class
{
T Get(object id);
IQueryable<T> GetAll();
void Insert(T entity);
void Delete(T entity);
void Save(T entity);
}
Then you can inject a repository into a controller:
public class MyController
{
private readonly IGenericRepository<Foo> _fooRepository;
public MyController(IGenericRepository<Foo> fooRepository)
{
_fooRepository = fooRepository;
}
}
This keeps UI free of any DB dependencies and makes testing easier; from unit tests you can inject any mock that implements IRepository. This also allows the repository to implement and switch between technologies like Dapper or Entity Framework without any client changes and at any time.
The above example used a generic repository, but you don't have to; you can create a separate interface for each repository, e.g. IFooRepository.
There are many examples and many variations of how repository pattern can be implemented, so google some more to understand it. Here is one of my favorite articles re. layered architectures.
Another note: For small projects, it should be OK to put queries directly into controllers...
I can't speak for dapper personally, but I've always restricted my db access to models only except in very rare circumstances. That seems to make the most sense in my opinion.
A little more info: http://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller
A model notifies its associated views and controllers when there has been a change in its state. This notification allows the views to produce updated output, and the controllers to change the available set of commands. A passive implementation of MVC omits these notifications, because the application does not require them or the software platform does not support them.
Basically, data access in models seems to be the standard.
I agree with #void-ray regarding the repository model. However, if you don't want to get into interfaces and dependency injection you could still separate out your data access layer and use static methods to return data from Dapper.
When I am using Dapper I typically have a Repository library that returns very small objects or lists that can then be mapped into a ViewModel and passed to the View (the mapping is done by StructureMap, but could be handled in the controller or another helper).
According to the MVC design pattern, if we create a user (database work) and we have to send a mail with an activation code to the user, would this fit in the model or in the controller, after the model created the database record?
The MVC pattern is used to create an abstraction between the business logic (the model) and the GUI (the view). The controller is just an adapter (google adapter pattern) between those two blocks.
Hence the controller should only have code which is used to fetch the required information from the controller and adopt it so it fits the view. Any other logic should be in the model.
That only make sense if you understand that the model is not a single class but all of your business logic.
Example (implementation specific, but I hope that you understand):
public class UserController : Controller
{
// notice that it's a view model and not a model
public ActionResult Register(RegisterViewModel model)
{
UserService service;
User user = service.Register(model.UserName);
return View("Created");
}
}
// this class is located in the "model"
public class UserService
{
public User Register(string userName)
{
// another class in the "model"
var repository = new UserRepository();
var user = repository.Create(userName);
// just another "model" class
var emailService = new EmailService();
emailService.SendActivationEmail(user.Email);
return user;
}
}
MVC and MVC-inspired design patterns are combination of two layers:
Presentation layer
Model layer
Presentation layer is made up from views, controllers and (mostly in web-oriented solutions) templates. This layer deals with user interaction. It recognizes user input, produces responses and governs other aspect of user interface. The controllers, based on user interaction, change the state of model layer.
The model layer deals with domain business rules and interacts with different forms of storage. Model layer, just like presentation layer, is no any single object or class, but a group of structures with different responsibilities.
In this case, it would make sense for the service, which deals with user management, to use the different structures, that would both send the verification email, create an account and store this newly created user.
Services in model layer act like the barrier, that isolated the presentation layer from business logic. They deal with interaction between domain objects and storage abstractions (data mappers, repositories, units of work, etc.).
TL;DR
Email, with activation code for the newly created user, should be sent from model layer.
The controller is an object which simplifies and delegates messages to the model objects.
What you will have is an Interface object (or boundary object) within your model that represents the link between two systems (your system and email). class EmailClient. Your model objects will collaborate with this object when required.
What is the difference between a MVC Model object, a domain object and a DTO?
My understanding is:
MVC Model object:
Models the data to be displayed by a corresponding view. It may not map directly to a domain object, i.e. may include data from one or more domain objects.
Client side
May contain business logic. Eg. validations, calculated properties, etc
No persistence related methods
Domain object:
An object that models real-world object in the problem domain like Reservation, Customer, Order, etc. Used to persist data.
Server side
No business logic
DTO (Data Transfer Object):
An object used to transfer data between layers when the layers are in separate processes, e.g. from a DB to a client app. Allows a single transaction across the wire rather than multiple calls when fetching data corresponding to multiple domain objects. A DTO contains just data and accessor methods and there is no logic present. The data is for a particular DB transaction, so it may or may not directly map to a domain object as it may include data from one or more domain objects.
Used on both server and client sides as it is passed between layers
No business logic
No persistence related methods
So, the questions:
Is above understanding correct? Am I missing some key points?
Are there any reasons not to use Domain objects as the MVC Model assuming that the Model objects do not require extra business logic?
Are there any reasons not to use DTOs as the MVC Model assuming that the Model objects do not require extra business logic?
Domain and model objects are essentially the same, and may contain business logic. Depending on implementation, domain and DTO objects may be equivalent if you remove business logic from the model into a service class.
Often a key variant of the DTO is the View Model, which is used purely to transfer data between the domain model and the view, although often a View Model may contain logic, although this should be purely UI logic.
The Domain and DTO can also be your "model" objects - you can have a view to render the details of the "Customer" domain object.
A domain object can have business logic to enforce the properties of the domain entity. validation is one such case. The domain object by itself does not contain persistence related methods, but it can have meta-data (like annotations) to support persistence
the POJO programming model makes it possible to use the same object as your domain, DTO and model objects - essentially, you will not be implemented any extraneous interfaces that will only apply to one layer but does not apply to others.
A DTO = is an object that carries data between processes.
But the most interesting part is that, it does not have any behavior except for storage and retrieval of its own data!!!
Sticking with the MVC methodology...
Domain = subject of your entire application.
Model = contains the (programming languages objects : EX: C# objects) to make up the universe of your application.
They can obvioussly have behaviour and properties(see difference with DTO).
Often an application (a light one) can have one model - case in which your model is exactly your domain.
Another model can be, a totaly different object type, that is processing another one. Both of them, in this case are part of your domain, and are named "domain models - objects".
Hopefully this answer is exhaustive and makes it all clear for you !
My understing (in a big short) is as follows:
(MVC) Model object:
represent some things in a some usage context eg. PersonEditModel, PersonViewModel or just PersonModel
has no business logic
can be subject of some valdation logic etc.
used to provide data from one application layer to another eg. MVC Controller <-> MVC View
Domain object:
represents some business object (real world object in the problem domain)
has business logic
do not allow invalid object state, has methods to properly change object's state
used to encapsulate business logic related to it
have not to be used to persist data (or even should not)
DTO (Data Transfer Object):
similar to Model object but should have flat structure
only simple type properties/fields (strings, numbers, datetimes, booleans)
used to transfer data cross application boundaries eg. between web server and web browser
Any definition for most of the objects is various based on place of using of objects:
Model: is a general definition for using object in client or server.
Model View : is a object using in client most of the time.
Domain Object : is a object using in server and transfering data to the database.
Data Transfer Object(DTO) : is a object that transfer data from one object to another object, specially in getting data in API Call(for example: in api GET Method call for getting data you must not to give database models to client, for this purpose you use dto).
Notice: the definitions are true most of the time but in some situations arent't practical.
MVC and DDD can be used together. What we call "Models" both in DDD and MVC are virtually the same: abstractions. Using pseudo-code we can illustrate a few examples.
Model View Controller (MVC)
The Model View Controller ARCHITECTURE separates the software into three parts:
The Model Layer
The Model layer from the MVC Architecture is where the logic resides. In this layer we have our models and business logic.
class Car {
String color;
String year;
Cat(color, year) {
this.color = color;
this.year = year;
}
//getters & setters
}
A simple Car abstraction.
class CarService {
save(car) {
if(car.getColor() != null && car.getYear() != null) {
methodToSave(car);
} else {
throwsException();
}
}
find(car) {
return methodToFind(car);
}
update(car) {
assertThatExists(car);
methodToSave(car);
}
delete(car) {
assertThatExists(car);
methodToDelete(car);
}
}
A simple CRUD for Car using a Service
The View Layer
The View layer is where the user interface resides. Here's where the user can interact with the system, which will then trigger the Controllers on actions performed, which will then inform the Model layer and request data. The View Layer can reside either in the client-side of the application or the server-side of the application (ie: JSF (Java Server Faces) as Server-Side, ReactJS as Client-Side). By any means, even if the View layer resides on the client side, the client will need to request the server-side for sending requests. This may be done by HTTP requests for a Web-Based Application.
<theCarPage>
<theCar>
getTheCarOnLoad();
</theCar>
</theCarPage>
A pseudo-page for the Car.
The Controller Layer
The Controller layer basically receives input from the View and then convert and send the data to the Model Layer and vice-versa.
class CarController {
#OnLoadingTheCarPage
getTheCarOnLoad() {
return theCar();
}
}
The method to load the Car.
Domain Driven Design (DDD)
Domain Driven Design is a concept:
DDD lays it's foundations in the concept that classes, class variables and class methods must match it's core business domain.
Domain Driven Design Resides into the "M"
In this case, when MVC is applied, the Domain Driven Design resides into the Model Layer of the MVC Architecture. As explained before, the Model Layer is where the Business Logic of the application resides.
Either if you have entities or not, they still are Models. A Model is just an abstraction of something in the real world. A cat can be a Model if abstracted:
class Cat {
String color;
String age;
Cat(color, age) {
this.color = color;
this.age = age;
}
//getters & setters
}
Simple Cat abstraction. It is a Model of Cat.
DDD Entities
In Domain Driven Design we have Entities, that are also classified as Models. The difference between them is that Entities are identifiable. If you have a class that is identifiable and can be persisted, then it's an Entity. An Entity still, is a Model.
#AnEntity
#ThisCanBePersisted
class Cat {
#ThisIsAnId
#ThisValueIncrementsAutomatically
#PersistentProperty
Long id;
#PersistentProperty
String color;
#PersistentProperty
String age;
Cat(color, age) {
this.color = color;
this.age = age;
}
//getters & setters
}
A simple Entity. An Entity is a Model.
Data Transfer Objects (DTO)
Data Transfer Objects have no logic inside them and the only use to them is to be containers for transferring data from one endpoint to another. Usually Enterprise Entities are not Serializable by nature, so we need a way to send only the data that we need to be sent to a client.
Since a Model could have sensible data or simply data we don't want to share in a request for a fetch, for instance, then considering our Cat Model, we could create a DTO that does not share the Cat's ID:
class CatDTO {
String color;
String age;
//getters & setters
}
A Data Transfer Object for Cat. We only need it's properties and something to get and set the properties. We don't want to share it's ID.
So if we, for instance, had to request a list of all cats from our client using REST, then we would request the endpoint that responds with our CatDTO instead of our Cat Entity:
[
Cat {
"color": "yellow",
"age": "1"
},
Cat {
"color": "black",
"age": "4"
}
]
And that would be all the data that our client could see.
1) No, It is a definition of ViewModel. MVC Model Object and Domain Object both are same.
2) Domain Models (Objects) are always present, business logic is optional
3) If there is no business logic in Domain Object then automatically it becomes DTO.