I have question about MVP. If presenter sends request to model for data. Does model check these data or does presenter have to do it? For example: are data ok etc. Thank you for answers.
I think there are a couple approaches you can take to validate your data: Either the Domain objects or a Service.
You model is your domain. In domain driven development, your domain should know how to validate itself. So, you might have a standard Validate method on either class in the model. That can get a little tricky, though, if you need to make a database call to do the validation, although you could require the relevant data to be passed to your validation method, or provide a delegate to get the data if it's necessary.
Alternatively, you could put all the validation in the Service later, which your presenter would be calling to retrieve and to persist the model. This would result in the so-called antipattern "anemic domain". But, if that fits your application and architecture best, it might be the correct choice.
I would caution against having the presenter do it. That's not really it's job and it does not get reused like the model and service.
Example 1:
A Model class "News" stores its text in two different languages (fields: en_text, jp_text). Usually it has text in only one language. Should I translate the text in callback before_save using the Google Translate API, or should I place this code in the Controller?
Example 2
A Model class "Payment". When the payment is going to be settled, the system must notify an external service about the successful settling of the payment. Where should this code be placed, Model or Controller?
The model is usually used to "get" or "set" data, so technically speaking, if your external service provides a service to "get" or "set" data, then yes.
Your Models should be literally the whole business logic of your application. Requirements for your Application mean that you should translate your text or notify some service on payment, don't they? That means that you should write it in your Model.
Controller is an entity that processes request parameters to some business logic actions. Controller should not contain such parts.
As I know, you are using Rails for now, so look at following links (this patterns are also helpful for non-Ruby programmers):
http://api.rubyonrails.org/classes/ActiveResource/Base.html
http://weblog.jamisbuck.org/2006/10/18/skinny-controller-fat-model - classics :)
Lets say I have an order which got updated on the UI with some values (they could be ok/not ok to ensure save)
1. How do we validate the changes made? Should the DTO which carries the order back to service layer be validated for completeness?
Once the validation is complete? How does the service return the validation errors? Do we compose a ReponseDTO object and return it like
ResponseDTO saveOrder(OrderDTO);
How do we update the domain entity order? Should the DTO Assembler take care of updating the order entity with the latest changes?
If we imagine a typical tiered' approach, ASP .NET on Web Server, WCF on Application Server.
When the Order form is updated with data on the web and saved. The WCF receives a OrderDTO.
Now how do we update the order from DTO? Do we use an assembler to update the domain object with changes from DTO? something like
class OrderDTOAssembler {
updateDomainObject(Order, OrderDTO)
}
I will try answer some of your questions from my experience and how I should approach your problem.
First I should not let DTO conduct any validations, but just plain POCO DTO's usually have different properties with specific datatypes, so some kind of validation is done. I mean you have to apply an integer street number and string for street name etc.
Second as you point out. Let a ORderDTOAssembler convert from OrderDTO to Order and vice versa. This is done in the application layer.
Third I would use Visitor pattern Validation in a Domain Driven Design like the example. The OrderService will use an IOrderRepository to save/update the order. But using the visitor-validation approach the OrderService vill call Order.ValidatePersistance (see link in example - but this is a extension method that is implemented in infrastructure layer since it has "db knowledge") to check its state is valid. If true, then we to IOrderRepository.Save(order).
at last, if Order.ValidatePersistance fails we get one or more BrokenRules messages. These should be returned to client in a ResponseDTO. Then cient can act on messages and take action. Problem here can be that you will have a ResponseOrderDTO messages but maybe (just came up with this now) all your ResponseDTO can inherit from ResponseBaseDTO class that expose necessary properties for delivering BrokenRule messages.
I hope you find my thoughts useful and good luck.
I'm slightly confused about what exactly the Model is limited to. I understand that it works with data from a database and such. Can it be used for anything else though? Take for example an authentication system that sends out an activation email to a user when they register. Where would be the most suitable place to put the code for the email? Would a model be appropriate... or is it better put in a view, controller, etc?
Think of it like this. You're designing your application, and you know according to the roadmap that version 1 will have nothing but a text based command line interface. version 2 will have a web based interface, and version 3 will use some kind of gui api, such as the windows api, or cocoa, or some kind of cross platform toolkit. It doesn't matter.
The program will probably have to go across to different platforms too, so they will have different email subsystems they will need to work with.
The model is the portion of the program that does not change across these different versions. It forms the logical core that does the actual work of whatever special thing that the program does.
You can think of the controller as a message translator. it has interfaces on two sides, one faces towards the model, and one faces towards the view. When you make your different versions, the main activity will be rewriting the view, and altering one side of the controller to interface with the view.
You can put other platform/version specific things into the controller as well.
In essense, the job of the controller is to help you decouple the domain logic that's in the model, from whatever platform specific junk you dump into the view, or in other modules.
So to figure out whether something goes in the model or not, ask yourself the question "If I had to rewrite this application to work on platform X, would I have to rewrite that part?" If the answer is yes, keep it out of the model. If the answer is no, it may go into the model, if it's part of the essential logic of the program.
This answer might not be orthodox, but it's the only way I've ever found to think of the MVC paradigm that doesn't make my brain melt out of my ear from the meaningless theoretical mumbo jumbo that discussions about MVC are so full of.
Great question. I've asked this same question many times in my early MVC days. It's a difficult question to answer succintly, but I'll do my best.
The model does generally represent the "data" of your application. This does not limit you to a database however. Your data could be an XML file, a web resource, or many other things. The model is what encapsulates and provides access to this data. In an OOP language, this is typically represented as an object, or a collection of objects.
I'll use the following simple example throughout this answer, I will refer to this type of object as an Entity:
<?php
class Person
{
protected $_id;
protected $_firstName;
protected $_lastName;
protected $_phoneNumber;
}
In the simplest of applications, say a phone book application, this Entity would represent a Person in the phone book. Your View/Controller (VC) code would use this Entity, and collections of these Entities to represent entries in your phone book. You may be wondering, "OK. So, how do I go about creating/populating these Entities?". A common MVC newbie mistake is to simply start writing data access logic directly in their controller methods to create, read, update, and delete (CRUD) these. This is rarely a good idea. The CRUD responsibilities for these Entities should reside in your Model. I must stress though: the Model is not just a representation of your data. All of the CRUD duties are part of your Model layer.
Data Access Logic
Two of the simpler patterns used to handle the CRUD are Table Data Gateway and Row Data Gateway. One common practice, which is generally "not a good idea", is to simply have your Entity objects extend your TDG or RDG directly. In simple cases, this works fine, but it bloats your Entities with unnecessary code that has nothing to do with the business logic of your application.
Another pattern, Active Record, puts all of this data access logic in the Entity by design. This is very convenient, and can help immensely with rapid development. This pattern is used extensively in Ruby on Rails.
My personal pattern of choice, and the most complex, is the Data Mapper. This provides a strict separation of data access logic and Entities. This makes for lean business-logic exclusive Entities. It's common for a Data Mapper implementation to use a TDG,RDG, or even Active Record pattern to provide the data access logic for the mapper object. It's a very good idea to implement an Identity Map to be used by your Data Mapper, to reduce the number of queries you are doing to your storage medium.
Domain Model
The Domain Model is an object model of your domain that incorporates behavior and data. In our simple phone book application this would be a very boring single Person class. We might need to add more objects to our domain though, such as Employer or Address Entities. These would become part of the Domain Model.
The Domain Model is perfect for pairing with the Data Mapper pattern. Your Controllers would simply use the Mapper(s) to CRUD the Entities needed by the View. This keeps your Controllers, Views, and Entities completely agnostic to the storage medium. This also allows for differing Mappers for the same Entity. For example, you could have a Person_Db_Mapper object and a Person_Xml_Mapper object; the Person_Db_Mapper would use your local DB as a data source to build Entities, and Person_Xml_Mapper could use an XML file that someone uploaded, or that you fetched with a remote SOAP/XML-RPC call.
Service Layer
The Service Layer pattern defines an application's boundary with a layer of services that establishes a set of available operations and coordinates the application's response in each operation. I think of it as an API to my Domain Model.
When using the Service Layer pattern, you're encapsulating the data access pattern (Active Record, TDG, RDG, Data Mapper) and the Domain Model into a convenient single access point. This Service Layer is used directly by your Controllers, and if well-implemented provides a convenient place to hook in other API interfaces such as XML-RPC/SOAP.
The Service Layer is also the appropriate place to put application logic. If you're wondering what the difference between application and business logic is, I will explain.
Business logic is your domain logic, the logic and behaviors required by your Domain Model to appropriately represent the domain. Here are some business logic examples:
Every Person must have an Address
No Person can have a phone number longer than 10 digits
When deleting a Person their Address should be deleted
Application logic is the logic that doesn't fit inside your Domain. It's typically things your application requires that don't make sense to put in the business logic. Some examples:
When a Person is deleted email the system administrator
Only show a maximum of 5 Persons per page
It doesn't make sense to add the logic to send email to our Domain Model. We'd end up coupling our Domain Model to whatever mailing class we're using. Nor would we want to limit our Data Mapper to fetch only 5 records at a time. Having this logic in the Service Layer allows our potentially different APIs to have their own logic. e.g. Web may only fetch 5, but XML-RPC may fetch 100.
In closing, a Service ayer is not always needed, and can be overkill for simple cases. Application logic would typically be put directly in your Controller or, less desirably, In your Domain Model (ew).
Resources
Every serious developer should have these books in his library:
Design Patterns: Elements of Reusable Object-Oriented Software
Patterns of Enterprise Application Architecture
Domain-Driven Design: Tackling Complexity in the Heart of Software
The model is how you represent the data of the application. It is the state of the application, the data which would influence the output (edit: visual presentation) of the application, and variables that can be tweaked by the controller.
To answer your question specifically
The content of the email, the person to send the email to are the model.
The code that sends the email (and verify the email/registration in the first place) and determine the content of the email is in the controller. The controller could also generate the content of the email - perhaps you have an email template in the model, and the controller could replace placeholder with the correct values from its processing.
The view is basically "An authentication email has been sent to your account" or "Your email address is not valid". So the controller looks at the model and determine the output for the view.
Think of it like this
The model is the domain-specific representation of the data on which the application operates.
The Controller processes and responds to events (typically user actions) and may invoke changes on the model.
So, I would say you want to put the code for the e-mail in the controller.
MVC is typically meant for UI design. I think, in your case a simple Observer pattern would be ideal. Your model could notify a listener registerd with it that a user has been registered. This listener would then send out the email.
The model is the representation of your data-storage backend. This can be a database, a file-system, webservices, ...
Typically the model performs translation of the relational structures of your database to the object-oriented structure of your application.
In the example above: You would have a controller with a register action. The model holds the information the user enters during the registration process and takes care that the data is correctly saved in the data backend.
The activation email should be send as a result of a successful save operation by the controller.
Pseudo Code:
public class RegisterModel {
private String username;
private String email;
// ...
}
public class RegisterAction extends ApplicationController {
public void register(UserData data) {
// fill the model
RegisterModel model = new RegisterModel();
model.setUsername(data.getUsername());
// fill properties ...
// save the model - a DAO approach would be better
boolean result = model.save();
if(result)
sendActivationEmail(data);
}
}
More info to the MVC concept can be found here:
It should be noted that MVC is not a design pattern that fits well for every kind of application. In your case, sending the email is an operation that simply has no perfect place in the MVC pattern. If you are using a framework that forces you to use MVC, put it into the controller, as other people have said.
In coding a traditional MVC application, what is the best practice for coding server-side form validations? Does the code belong in the controller, or the model layer? And why?
From Wikipedia:
Model-view-controller (MVC) is an architectural pattern used in software engineering. Successful use of the pattern isolates business logic from user interface considerations, resulting in an application where it is easier to modify either the visual appearance of the application or the underlying business rules without affecting the other. In MVC, the model represents the information (the data) of the application and the business rules used to manipulate the data; the view corresponds to elements of the user interface such as text, checkbox items, and so forth; and the controller manages details involving the communication to the model of user actions such as keystrokes and mouse movements.
Thus, model - it holds the application and the business rules.
I completely agree with Josh. However you may create a kind of validation layer between Controller and Model so that most of syntactical validations can be carried out on data before it reaches to model.
For example,
The validation layer would validate the date format, amount format, mandatory fields, etc...
So that model would purely concentrate on business validations like x amount should be greater than y amount.
My experience with MVC thus far consists of entirely rails.
Rails does it's validation 100% in the Model.
For the most part this works very well. I'd say 9 out of 10 times it's all you need.
There are some areas however where what you're submitting from a form doesn't match up with your model properly. There may be some additional filtering/rearranging or so on.
The best way to solve these situations I've found is to create faux-model objects, which basically act like Model objects but map 1-to-1 with the form data. These faux-model objects don't actually save anything, they're just a bucket for the data with validations attached.
An example of such a thing (in rails) is ActiveForm
Once the data gets into those (and is valid) it's usually a pretty simple step to transfer it directly across to your actual models.
The basic syntax check should be in the control as it translates the user input for the model. The model needs to do the real data validation.