I'm trying to get started with the repository pattern and ASP.NET MVC, and I can't help but believe I'm missing something. Forgive me if this is a stupid question, but it seems to me like an implementation violates DRY exponentially. For example, in my (admittedly novice) understanding in order to implement this, I would have to:
Create my database model (Currently using Linq to Sql)
Create a IRepository for each concept (table or group of related tables)
Create an implementation for each IRepository
Do we return L2S objects or some sort of DTO?
Create viewmodels which either are containers or copies of the data
Use some method of DI (Windsor or Unity?) on the controllers
While I realize scalability and portability come at an expense, it just feels like I'm missing something?
I tried to implement the Repository Pattern in LINQ 2 SQL and it doesn't work very well, mainly because L2S doesn't use POCOs and you have to map to DTOs all the time as you mention. Although you could use something like AutoMapper, L2S just isn't a very good fit for the Repository Pattern.
If you're going to use the Repository Pattern (and I would recommend it), try a different data access technology such as NHibernate or Entity Framework 4.0's POCO support.
Also you wouldn't create a Repository for each and every table, you create a Repository per Domain Aggregate, and use the Repository to access the Aggregate's Root entity only. For instance, if you have an e-commerce app, with Order and OrderItem entities, an Order has one-or-many OrderItems. These 2 entities are part of a single Aggregate, and the Order entity is the Aggregate Root. You'd only create an OrderRepository in this case, NOT an OrderItemRepository as well. If you want to add new OrderItems you'd do so by getting a reference to the Order entity, then adding the new OrderItem to the Order's Items collection, then saving the Order using your OrderRepository. This technique is called Domain Driven Design, and it's a very powerful paradigm to use if you have a complex Domain Model and business rules in your application. But it can be over kill in simple applications, so you have to ask yourself does the complexity of your Domain Model warrant using this approach.
In terms is adhering to DRY, normally I create a base Repository class that has common methods for Save, Delete, FetchById, that sort of thing. As long as my Repository classes implement this base class (OrderRepository, ProductRepository etc.) then they get these methods for free and the code is DRY. This was easy to do in NHibernate because of POCO support, but impossible to do in Linq 2 SQL.
Don't worry too much about sending your Domain Models directly to the view, most dedicated ViewModels look almost identical to the Domain Model anyway, so what's the point. Although I tend to avoid using the DM for posting data back to the server because of under/overposting security concerns.
If you follow this POCO approach (and ditch LINQ 2 SQL, honestly!!), you end up with only one class (your POCO entity) instead of 3 (L2S class, DTO and ViewModel).
It is possible to implement the Repository Pattern badly, so tread carefully, read a few tutorials, blog posts books etc. (I recommend Steven Sanderson's book, especially look at the Pre-Requisites chapter) But once mastered, it becomes a very powerful way to organise the complexity of hydrating Model objects to and from a data-store. And if you use Repository interfaces (IOrderRepository etc.) and have them injected via an IOC Container, you also gain the benefits of maintainability and unit testability.
Do you understand why your doing these things or are you just following along with a blog article or other source?
Don't implement the Repository pattern because its the new hotness. Implement it because you understand how these separation of concerns helps your project and overall quality of your code.
From your ?'s in your question it sounds like you need to do some more reading before you implement. Your probably missing a meaningful understanding of the overall architecture approach. Please don't take this the wrong way and I'm not trying to be negative.
Side Rant:
Obviously something is missing from the newest repository hotness picture because the confusion about implementation details like single Repository vs. Many/Grouped "DTO or not to DTO" are just so ambiguous and subjective. This is a "nickle question" that pops up again and again.
This has been brought up before, at first glance certain aspects of properly separating concerns does seem to violate DRY.
As you've mentioned MVC have you read Steve Sanderson's Pro ASP.NET MVC 2 Framework book? It spends a great deal of time explaining why using the repository pattern is a good idea.
You might find that, for the projects you're working on, it isn't appropriatte, that's okay. Don't use it and see if you come across problems that this could have addressed. You don't need to be a developer for long to realise how crucial it is to keep different parts of your application as loosely coupled as possible.
Related
I'm aware that in model-view-controller, the Model is the class part.
If I have a User class and instantiate an object, the object must refer to a single user from the database.
So I'll have the CRUD methods on the user, for that specific user.
But if I need a function to run a SELECT * FROM Users, should I create a function within the User class? Or a function in a helper file? Or in the controller? Where should it go, in order to respect the MVC pattern?
I mean, it makes no sense to instantiate a User object just to run a function to display the Users table.
I'm not sure if this will raise "primarily opinion based" flags. I just don't know where those functions should go. If you guys consider the question worth closing, it's ok. But tell me in the comments in which stack community I should ask this.
Back up a bit. Let's go foundational for a moment.
In the MVC pattern
The model is your state (in simple terms), meaning a representation of the data important to the business functionality you are working with
The view is a way of presenting the state to the user (NOTE user here could be another system, as you can use MVC patterns for service endpoints)
The controller ensures the model gets to the view and back out of the view
In a system designed with good separation of state, business functions are not present in the model, the view or the controller. You segregate business functionality into its own class library. Why? You never know when the application will require a mobile (native, not web) implementation or a desktop implementation or maybe even become part of a windows service.
As for getting at data, proper separation of concerns states the data access is separate not only from the model, view and controller, but also from the business functionality. This is why DALs are created.
Before going on, let's go to your questions.
should I create a function within the User class? This is an "active record" pattern, which is largely deprecated today, as it closely couples behavior and state. I am sure there are still some instances where it applies, but I would not use it.
Or a function in a helper file? Better option, as you can separate it out. But I am not fond of "everything in a single project" approach, personally.
Or in the controller? Never, despite Scott Gu's first MVC examples where he put LINQ to SQL (another groan?) in the controller.
Where should it go, in order to respect the MVC pattern?
Here is my suggestion:
Create a DAL project to access the data. One possible pattern that works nicely here is the repository pattern. If you utilize the same data type for your keys in all/most tables, you can create a generic repository and then derive individual versions for specific data. Okay, so this one is really old, but looking over it, it still has the high level concepts (https://gregorybeamer.wordpress.com/2010/08/10/generics-on-the-data-access-layer)
Create a core project for the business logic, if needed. I do this every time, as it allows me to test the ideas on the DAL through unit tests. Yes, I can test directly in the MVC project (and do), but I like a clean separation as you rarely find 0% business rules in a solution.
Have the core pull the data from the DAL project and the MVC project use the core project. Nice flow. Easy to unit test. etc.
If you want this in a single project, then separate out the bits into different folders so you can make individual projects, if needed, in the future.
For the love of all things good and holy, don't use the repository pattern. #GregoryABeamer has a great answer in all respects, except recommending you create repository instances to access your entities. Your ORM (most likely Entity Framework) covers this, and completely replaces the concepts of repositories and unit of work.
Simply, if you need something from your database, hit your ORM directly in your controller. If you prefer, you can still add a level of abstraction to hide the use of the ORM itself, such that you could more easily switch out the data access with another ORM, Web Api, etc. Just don't do a traditional unit of work with tens or hundreds of repository instances. If you're interested, you can read a series of posts I wrote about that on my blog. I use the term "repository" with my approach, but mostly just to contrast with the typical "generic" repository approach you find scattered all over the interwebs.
I'd use some kind of 'Repository' layer. Then, my controller calls the UserRepository GetAll method and sends the data to View layer.
I am building ASP.NET MVC 5 application.
I read about Repository and Unit of Work (UoW) Patterns here.
These examples use Entity Framework which adds a high-level of abstraction itself.
I am using ADO.NET and not EF. I want to know:
Whether Repository and UoW patterns makes any sense with ADO.NET?
How will my Repositories and UoW look with ADO.NET? Any Samples?
Can I add a separate class library for Repository or to make it a part of DAL?
I've written a blog post which teaches you on how to write driver independent code and how to implement Uow/Repository pattern with plain ADO.NET.
It's a bit too long to include in this answer, but basically the IDbtransaction will represent your Unit oF Work (or be contained in one) and every repository will take the transaction or UoW in it's constructor.
To create a command using a IDbTransaction
using (var cmd = transaction.Connection.CreateCommand())
{
cmd.Transaction = transaction;
//do a CRUD operation here.
}
If you look at the definitions of the patterns, and the patterns required to support them, you'll see that, if you start to implement them yourself, you'll never be wandering far from creating your own ORM. While this is a fascinating task, it's never worth it when you consider NHibernate and EntityFramework.
However, to answer your question, I found Fowler's PoEAA book invaluable in learning how to write my own UoW, DataMappers and Repositories, all based on ADO.Net. It's written by someone who's obviously done it for real, made all the mistakes and then documented them so that you don't have to. I haven't read the article you've linked, however I'm often wary of using articles like this as they only demonstrate a surface-level consideration of patterns like this.
The Unit of Work pattern is more important when you're talking about standard ADO.NET because you have to make absolutely sure that the connections you're opening are only open for the required period of time, achieved by wrapping connections inside using statements.
Unit of Work in ADO.NET would look something like this:
using (SqlConnection con = new SqlConnection(//connection string)
{
using (SqlCommand cmd = new SqlCommand(storedProcname, con))
{
//...
}
}
As you're using using statements to encompass your unit of work, you can be assured that under the hood SqlConnection.Dispose() calls the SqlConnection.Close() method, and SqlCommand.Dispose() calls SqlCommand.Close().
As answered in your previous question, you can separate these two out if you wish to do so, but personally I think they should be the same thing.
Some time ago I wrote a blog post why that very tutorial you've linked is harmful . tldr; a repository uses a DAO implementing UoW, but the repository should not be part of a UoW. Unless you like to complicate your code base/life.
To answer your questions:
Once you're using EF or any other ORM, UoW is automatically implemented there. If you go the micro-ORM path (there is no valid reason to use ado.net directly) , the UoW is basically the db transaction. A repository should always deal with application objects, never with ORM (persistence) objects. If your app objects are used as persistence entities, then you probably have a standard CRUD app, and you don't really need the repository pattern. For simple apps, use the ORM directly (it saves a lot of time).
A Repository makes use of a DAO implementing UoW as an implementation detail. The rest of the app isn't aware of anything outside the repository (interface) itself.
A repository interface is defined where it's used (usually a the Domain/Business layer). The repository implementation is part of the DAL. Note that you should use repositories only for changing the model (create/update/delete). For queries, it's much easier and maintainable to have query services (objects) handling a specific use cases and which work with the db directly.
There's a a LOT of misusing the repository pattern out there, I suggest reading my "Repository for dummies" post to understand that's a very simple pattern which has nothing to do with the complicated examples you're usually seeing.
I'd like to create a good app in ASP.NET MVC 5 using EF 6 Code first concept. I want it to be well-designed i.e. having generally speaking: Presentation, Logic and Data layers separated. I want it to be testable :)
Here's my idea and some issues related with creating application
Presentation layer: It's my whole MVC - view models(not models), views, controllers
I believe that's validation should be done somewhere else (in my opinion - it's a part of business logic) but it's quite convenient to use attributes from the DataAnnotations namespace in ViewModelds and check validation in controller.
Logic layer: Services - classes with their interfaces to rule business logic.
I put there functions like: AddNewPerson(PersonViewModel Person), SendMessageToPerson(...).
They will use DB context to make their actions (there's a chance that not all of them will be relying on context). There's a direct connection between service and db - I mean the service class have reference do context.
Where should I do mapping between ViewModel and Model? I've heard that service is a bad place for it - so maybe in controllers. I've heard that service should do the work related with db exclusively.
Is it right? Is my picture of service layer is good?
Data layer: I've read about Repository and UoW patterns a lot. There're some articles which suggest that EF6 implements these two things. I don't want to create extra code if there's no need for such a behavior. The question is: am i right to assume that i don't need them?
Here's my flow:
View<->Controllers(using ViewModels)<->Services(using Models)<->DB.
**I'm gonna use DI in my project.
What do you think about my project structure?
There is no reason to use a Unit of Work pattern with Entity Framework if you have no need to create a generic data access mechanism. You would only do this if you were:
using a data access technology that did not natively support a Unit of work pattern (EF does)
Wanted to be able to swap out data providers sometime in the future.. however, this is not as easy as it might seem as it's very hard NOT to introduce dependencies on specific data technologies even when using an Unit of Work (maybe even BECAUSE you are)... or
You need to have a way of unifying disparate data sources into an atomic transaction.
If none of those are the case, you most likely don't need a custom Unit of Work. A Repository, on the other hand can be useful... but with EF6 many of the benefits of a Repository are also available since EF6 provides mocking interfaces for testing. Regardless, stay away from a generic repository unless it's simply an implementation detail of your concrete repositories. Exposing generic repositories to your other layers is a huge abstraction leak...
I always use a Repository/Service/Façade pattern though to create a separation between my data and business (and UI and business for that matter) layers. It provides a convenient way to mock without having to mock your data access itself and it decouples your logic from the specific that are introduced by the Linq layer used by EF (Linq is relatively generic, but there are things that are specific to EF), a façade/repository/server interface decouples that).
In general, you're on the right path... However, let me point out that using Data Attributes on your view models is a good thing. This centralizes your validation on your model, rather than making you put validation logic all over the place.
You're correct that you need validation in your business logic as well, but your mistake is the assumption that you should only have it on the business logic. You need validation at all layers of your application.. And in particular, your UI validation may have different requirements than your business logic validation.
For instance, you may implement creating a new account as a multi-step wizard in your UI, this would require different validation than your business layer because each step has only a subset of the validation of the total object. Or you might require that your mobile interface has different validation requirements from your web site (one might use a captcha, while the other might use a touch based human validation for instance).
Either way, it's important to keep in mind that validation is important both at the client, server, and various layers...
Ok, let’s clarify a few things...
The notion of ViewModel (or the actual wording of ViewModel) is something introduced by Microsoft Martin Fowler. In fact, a ViewModel is nothing more than a simple class.
In reality, your Views are strongly typed to classes. Period. To avoid confusion, the wording ViewModel came up to help people understand that
“this class, will be used by your View”
hence why we call them ViewModel.
In addition, although many books, articles and examples use the word ViewModel, let's not forget that it's nothing more than just a Model.
In fact, did you ever noticed why there is a Models folder inside an MVC application and not a ViewModels folder?
Also, ever noticed how at the top of a View you have #model directive and not # viewmodel directive?
That's because everything could be a model.
By the way, for clarity, you are more than welcomed to delete (or rename) the Models folder and create a new one called ViewModels if that helps.
Regardless of what you do, you’ll ultimately call #model and not #viewmodel at the top of your pages.
Another similar example would be DTO classes. DTO classes are nothing more than regular classes but they are suffixed with DTO to help people (programmers) differentiate between all the other classes (including View Models).
In a recent project I’ve worked on, that notion wasn’t fully grasped by the team so instead of having their Views strongly typed to Models, they would have their Views strongly typed to DTO classes. In theory and in practice everything was working but they soon found out that they had properties such as IsVisible inside their DTO’s when in fact; these kind of properties should belongs to your ViewModel classes since they are used for UI logic.
So far, I haven’t answered your question but I do have a similar post regarding a quick architecture. You can read the post here
Another thing I’d like to point out is that if and only if your Service Layer plans on servicing other things such as a Winform application, Mobile web site, etc...then your Service Layer should not be receiving ViewModels.
Your Service Layer should not have the notion of what is a ViewModel. It should only accept, receive, send, etc... POCO classes.
This means that from your Controller, inside your ActionResult, once the ModelState is Valid, you need to transform your ViewModel into a POCO which in turn, will be sent to the method inside your Service Layer.
In other words, I’d use/install the Automapper nugget package and create some extension methods that would convert a ViewModel into a POCO and vice-versa (POCO into a ViewModel).
This way, your AddNewPerson() method would receive a Person object for its parameter instead of receiving a PersonViewModel parameter.
Remember, this is only valid if and only if your Service Layer plans on servicing other things...
If that's not the case, then feel free to have your Service Layer receive, send, add, etc...ViewModels instead of POCOs. This is up to you and your team.
Remember, there are many ways to skin a cat.
Hope this helps.
I am learning EF Code First and I am struggling a bit with what patterns to use in my application. I have read many conflicting sugestions and arugments some stating you should use the Repository pattern while others say doing so is redundant, which I tend to agree.
Here is my delima:
Suppose I am building a REST Web Service that is going to allow me to manage customers. This service will allow me to add customers, delete customes, and edit customers, and find customers.
Should I:
A.) My question comes down to where should my business logic go. Should I have a CustomerManager class that provides Add, Edit, Delete, and Find methods that take in a Customer entity? Should my validation logic go in those methods?
B.) Should I use an Active Record style of development when my Customer entity would have Save(), Delete(), and Find() methods on it withall validations login being done inside of the Customer class?
C.) Should I do some type of hybrid, where simple validation logic is on the entity itself. This could be done through code first attributing. I could also have a simple save method on the entity. Then, I could do complex business validation logic, deletes(), finds(), and multi-entity saves in a CustomerManager class?
I kind of lean toward option C. In the past I have typically used Manager/Service classes keeping my entities pretty simple. However, since code first does entity property validation on the entity level, it seems like maybe all simple entity validation should go there.
I realize this could be somewhat of a religious topic, but I would like to get some other options on what would be the best way to put together a solid application.
EF 4.1 Code first combine unit of work with data mapper pattern.
So, I would not recommend use active record pattern.
Repository pattern with entity framework is common solution. If you want some simple validation logic, you can use DataAnnotations which works with entity framework well.
Here is simple example of implement repository pattern with EF :
http://www.efekaptan.com/repository-pattern-with-entity-framework-code-first-4.1
Is there a name for the software design pattern that involves MVC with domain models and view models? It's the pattern used when a tool like AutoMapper is employed.
I was attempting to explain the advantages of such a design to some fellow programmers and was calling it MVVM but I'm now of the opinion that's not right and the MVVM pattern is where the view stage is refining the model for its own purposes rather than changing what's passed between the Controller and the View.
Of course it all gets confusing with the number of M's and V's and rather than getting tied up in more knots I thought I'd ask the experts.
Its called MVC. No, seriously. Your website's models just happen to be called ViewModels.
MVC doesn't say what the model is supposed to be, just how its supposed to interact with M and C.
As you might have read or experienced that Automapper is used to map objects. When it comes to MVC, the V (View) and C (Controller) are pretty clear. What confuses many people is the M (Model). MVC doesn't lay strong emphasis on how model should be contructed. You can construct your models by running a direct SQL query or using an ORM tool like NHibernate or LINQ2SQL or Entity Framework of your choice.
If you really want to separate the concerns you can go a step ahead and do what most people do. You can introduce repository pattern to handle model data which can be retrieved using a service. This is the situation where Automapper would come into the picture where you need to map entities and Dto's.
I don't think there is any need to abbriviate this method of building models from service layer. The normal MVC stands valid in this case as well :)
The "Related" questions provided me with the best answer I think I'm going to get.
The design pattern is defined fairly well in the article How we do MVC – View models and the appropriate monica seems to be MVC-ViewModels or MVC-VM.