In one of my views, I have a ViewModel which I populate from two tables, and then bind a List<ViewModel> to an editable GridView (ASP.NET Web Forms).
Now I need to send that edited List<ViewModel> back to the Services layer to update it in the database.
My question is - is it Okay to send the ViewModel back to Services, or should it stay in the Presentation? If not - should I better use a DTO? Many thanks.
Nice question !
After several (hard) debates with my teammates + my experience with MVC applications, I would not recommend to pass viewmodel to your service / domain layer.
ViewModel belongs to presentation, no matter what.
Because viewModel can be a combination of different models (e.g : 1 viewModel built from 10 models), your service layer should only work with your domain entities.
Otherwise, your service layer will end up to be unusable because constrained by your viewModels which are specifics for one view.
Nice tools like https://github.com/AutoMapper/AutoMapper were made to make the mapping job.
I would not do it. My rule is: supply service methods with everything they need to do their job and nothing more.
Why?
Because it reduces coupling. More often than not service methods are addressed from several sources (consumers). It is much easier for a consumer to fulfil a simple method signature than having to build a relatively complex object like a view model that it otherwise may have nothing to do with. It may even need a reference to an assembly it wouldn't need otherwise.
It greatly reduces maintenance effort. I think an average developer spends more than 50% of his time inspecting and tracking existing code (maybe even much more). Now everybody knows that looking for something that is not there takes disproportionally much time: you must have been everywhere to be sure. If a method receives arguments (or object with properties) that are not used directly or further down the call stack you or others will walk this long road time and again.
So if there is anything in the view model that does not play a part in the service method, don't use it to call the method.
Yes. I am pretty sure it is ok.
Try to use MS Entity Framework and it will help you allots.
Related
My question might seem strange for pros but please take to account that I am coming from ruby on rails world =)
So, I am learning ASP.NET Core. And I like what I am seeing in it compared to rails. But there is always that but... Let me describe the theoretical problem.
Let's say I have a Product model. And there are over 9000 records in the database. It is obvious that I have to paginate them. I've read this article, but it seems to me that something is wrong here since the controller shouldn't use context directly. It has to use some repository (but that example might be provided in such a way only for simplicity).
So my question is: who should be responsible for pagination? Should it be the controller which will receive some queryable object from the repository and take only those records it needs? Or should it be my own business service which does the same? Or should the repository has a method like public IEnumerable<Product> ListProducts(int offset, int page)?
One Domain-Driven-Design solution to this problem is to use a Specification. The Specification design pattern describes a query in an object. So you might create a PagedProduct specification which would take in any necessary parameters (pageSize, pageNumber, filter). Then one of your repository methods (usually a List() overload) would accept an ISpecification and would be able to produce the expected result given the specification. There are several benefits to this approach. The specification has a name (as opposed to just a bunch of LINQ) that you can reason about and discuss. It can be unit tested in isolation to ensure correctness. And it can easily be reused if you need the same behavior (say on an MVC View action and a Web API action).
I cover the Specification pattern in the Pluralsight Design Patterns Library.
For first, I would like to remind you that all such examples you linked are overly simplified, so it shouldn't drive you to believe that that is the correct way. Simple things, with fewer abstraction layers are easier to oversee and understand (at least in the case of simple examples for beginners when the reader may not know where to look for what) and that's why they are presented like that.
Regarding the question: I would say none of the above. If I had to decide between them then I would say the service and/or the repository, but that depends on how you define your storage layer, etc.
"None of the above", then what? My preference is to implement an intermediary layer between the service layer and the Web UI layer. The service layer exposes manipulation functionality but for read operations, exposes the whole collection as an IQueryable, and not as an IEnumerable, so that you can utilize LINQ-to-whatever-storage.
Why am I doing this, many may ask. Because almost all the time you will use specialized viewmodels. To display the list of products on an admin page, for example, you would need to display values of columns in the products table, but you are very likely to need to display its category as well. Very rarely is it the case that you need data only from one table and by exposing the items as an IQueryable<T> you get the benefit of being able to do Selects like this:
public IEnumerable<ProductAdminTableViewModel> GetProducts(int page, int pageSize)
{
backingQueryable.Select(prod => new ProductAdminTableViewModel
{
Id = prod.Id,
Category = prod.Category.Name, // your provider will likely resolve this to a Join
Name = prod.Name
}).Skip((page - 1) * pageSize).Take(pageSize).ToList();
}
As commented, by using the backing store as an IQueryable you will be able to do projections before your query hits the DB and thus you can avoid any nasty Select N+1s.
The reason that this sits in an intermediary layer is simply you do not want to add references to your web project neither in your repo nor in your service layer (project) but because of this you cannot implement the viewmodel-specific queries in your service layer simply because the viewmodels cannot be resolved there. This implies that the viewmodels reside in this same project as well, and to this end, the MVC project only contains views, controllers and the ASP.NET MVC-related guttings of your app. I usually call this intermediate layer as 'SolutionName.Web.Core' and it references the service layer to be able to access the IQueryable<T>-returning method.
I have been reading about entity framework over the past couple of days and have managed to get a fair idea of using it but I still have a couple of questions some of which might seem a bit too basic. For perspective I am using entity framework 4.0 in an asp.net web application.If you can answer any of the questions please go ahead.
What advantage do I get by using POCO templates. I understand that if I wish to get persistence ignorance and keep my Entities clear of any information related to storage POCO entities are the way to go. Also I could switch from Entity framework to say NHibernate with relative ease when using POCO entities? Apart from loose coupling is there any significant reason for me to go towards POCO entities. Also if I do use POCO do I end up losing anything. I still get change tracking and lazy loading with the help of proxies?
Is it normal practice to use the Entities of the EF model as Data transfer Objects or Business Objects. i.e for example I have a separate class library for my entity model.Supposing I am using MVP , where I want a list of Employee's in a company. The presenter would request my business logic functions which would query the entity model for the list of Employee's and return the list of entities to the presenter. In this case my presenter would need to have a reference to the EF model. Is this the correct way? In the case of my asp.net web applciation it shouldnt be a problem but if I am using web services how does this work? Is this the reason to go towards POCO entities?
Supposing The Employee entity has a navigation property to a company table. If I use and wrap the data context in an 'using' block , and try to access the navigation property in the BL I am assuming I would get an exception. Would I also get an exception if I turned off lazyloading and used the 'include' linq query to get the entity? On a previous post someone recommended I use an context per request implying that the context remains active even when I am in the BL. I am assuming I would still need to detach the object and attach it to the context on my next request if I wish to persist any changes I make? or Instead should I just query for the object again with the new context and update it?
This question has more to do with organizing files/best practices and is a followup to a question i posted earlier. When I am using separate files based on entities to organize my data access layer, what is the best practice to organize my queries involving joins between multiple tables. I am still a bit hazy on organization. Have tried searching online but havent had much help.
Terrific question. My first recommendation is to think in patterns. With that said...
You pretty much nailed the advantages of using POCO. There are some distinct advantages to decoupling your business objects (POCO entities) from your data access layer. But the primary reason is like you said the ability to change or modify layers below. However using POCO you are essentially following the Code First (CF) approach. Personally, I consider it Code In Parallel depending upon your software development life cycle. You still have all the bells and whistles that data or model first approach have and some since you can extend the DbContext which is ObjectContext under the hood. I read an article, which I cannot seem to find, that CF is the future of Entity Framework. Lastly the nice thing with POCO is you are able to incorporate validation rules here or else where. You can also provide projections. Lets say you have Date of Birth but you want an Age property as well. That now becomes a no brainer as the Age property is ignored when mapping to the database.
Personally I create my own business objects (POCO) for large projects that tend to have a life of its own where change is a way of life. Another thought is scalability and maintainability. What if down the road I choose to split functionality between applications where, like you mentioned web services, functionality is now delivered from two disparate locations. If you have encapsulated your business objects and DAL within the same code block separation or scalability has now become a bit more complex. However, consider the project. It may be small with very little future change so no need to throw a grenade to kill a fly. At which time data first might be the way to go and let edmx file represent your objects. So don't marry yourself to one technology or one methodology/pattern. Do what makes sense for your time and business.
Using statements are perfectly fine. In fact I've recently been turned on to then wrapping that within a TransactionScope. If an error occurs rollbacks are inherent. Next, something to consider is the UnitOfWork. UnitOfWork pattern encapsulates a snapshot of what needs to be performed where the Data Context is the boundaries from which you work within. For each UnitOfWork you have a subject for which work is to be performed on. For example an Employee. So if you are to save Employee information to keep it simple you would make a call to the BL service or repository (which ever). There you pass in the Employee Id, perform some work under that UnitOfWork where it is either instantiated in the constructor or using Dependency Injections (DI or IoC). Easy starter is StructureMap. There the service makes the necessary calls to your UnitOfWork (DbContext) then returns control back upstream (e.g. UI).
The best way to learn here is to view others code. I'd start with some Microsoft examples. I'd start with Nerd Dinner (http://nerddinner.codeplex.com/) then build off that.
Additional Reading:
Use prototype pattern or not
http://weblogs.asp.net/manavi/archive/2011/05/17/associations-in-ef-4-1-code-first-part-6-many-valued-associations.aspx
[EDIT]
NightHawk457, I'm terribly sorry for not responding to your questions. Hopefully you figured it out but for future readers...
To help everyone visualize, imagine the below Architecture using the Domain Model and Repository as an example. Remember, there are many ways to skin a cat so take this and make it your own and don't forget my Grenade comment above.
Data Layer (Data Access): MyDbContext : DbContext, IUnitOfWork, where IUnitWork contracts the CRUD operations.
Data Repository (Data Access / Business Logic): MyDomainObjectRepository : IMyDomainObjectRepository, which receives IUnitOfWork by Factory class or Dependency Injection. Calls MyDomainObject validation on CRUD operations.
Domain Model (Business Logic): MyDomainObject using [Custom] Validation Attributes. Read this for pros/cons.
MVVM / MVC / WCF (Presentation / Service Layers): What ever additional layers you chose, you now have access to your data which is wrapped nicely in smaller modules who are self encapsulating of their function. The presentation layer (e.g. ViewModel, Controller, Code-Behind, etc.) can then receive an IMyObjectRepository by a Factory class or by Dependency Injection.
Tips:
Pass connection string into MyDbContext so you can reuse MyDbContext.
MySql does not play well with System.Transactions.TransactionScope, example. I don't recall exactly but it was something MySql did not support. This makes Testing a bit difficult since we have created this level of separation.
Create a Test project for each layer and at the minimum test general functionality/rules.
Each Domain Object should extend base object with ID field at minimum. Also do not implement Key attributes here. Domain Object should not describe architecture but rather the specific data as an entity. Even on Code First this can be achieved by the Fluent API.
Think generics when creating MyDbContext. ;) Read Diego's post.
In ASP.NET, the repositories are nice to use with ObjectDataSources.
As you can see, there is clear separation of roles where IUnitOfWork and IMyDomainObjectRepository are the Interfaces which expose the above layers functionality. And as an example, IUnitOfWork could be NHibernate, Entity Framework, LinqToSql or ADO.NET where a change to the factory class or dependency injection registration is all that has to change. FYI, I've heard the Repository called the Service Layer as well. Personally I like the first name to not be confused with Web Services. The next big take away from this structure is realizing the scope for you Database Context (IUnitOfWork). A simple example would be a ASP.NET page where for each page there is one and only one IUnitOfWork for either each repository or for that scope of work. Same holds true for ViewModels, Controllers, etc. So let's say you need to utilize two repositories, EmployeeRepository and HRRepository. You then could share the IUnitOfWork between both or not. To cross page, ViewModel or Controller boundaries, we use the ID for entities where they are then pulled from the DB and work is performed. You could alternatively pass a DTO across boundaries and attach to the context but then you begin losing separation of layers.
To continue, POCO classes do not have to be auto generated. In fact you can create your Entity Classes from scratch and perform the mapping in your extended DbContext class inside the OnModelCreating(DbModelBuilder mb) method. Start here, then here and note the Additional Resources, google Fluent API and read this post by Diego.
As for validation, this is an interesting point because it would be GREAT if all Business Rules could be validated in one location. Well, as we all know that doesn't work real well. So here is my recommendation, keep all data level validation (i.e. required, range, format, etc.) with data annotation as much as possible in the domain object and leave process validation in the Repository with clear roles of the Repository (i.e. if (isEmployee) do this, else that). I say clear, such that you do not want to add an Employee in two different Repositories where validation has to be duplicated. To call the validation, start here. Capture the ValidationResults and send upstream with a MyRepositoryValidationException which contains a collection of validations errors (e.g. Employee is required) which can be presented to the presentation layer. With all that said, don't forget to perform validation at the presentation layer. You don't want post backs to make sure an Employee has a valid Email, for example.
Just remember to balance time and effort with complexity. For something simple, use Data First or Model First with your EDMX file. Then lay a repository on top of that which also contains all the validation rules.
I am currently working on several flex projects, that have gone in a relative short amount of time from prototype to almost quite large applications.
Time has come for some refactoring to be done, so obviously the mvc principle came into mind.
For reasons out my control a framework(i.e. robotlegs etc.) is not an option.
Here comes the question...what general guidelines should I take into consideration when designing the architecture?
Also...say for example that I have the following: View, Ctrl, Model.
From View:
var ctrlInstance:Ctrl= new Ctrl();
ctrl.performControllerMethod();
In Controller
public function performControllerMethod():void{
//dome some sort of processing and store the result in the model.
Model.instance.result = method_scope_result;
}
and based on the stored values update the view.
As far as storing values in the model, that will be later used dynamically in the application, via time filtering or other operation, everything is clear, but in cases where data just needs to go in(say a tree that gets populated once at loading time), is this really necessary to use the view->controller->model->view update scheme, or can I just make the controller implement IEventDispatcher and dispatch some custom events, that hold necessary data, after the controller operation finished.
Ex:
View:
var ctrlInstance:Ctrl= new Ctrl();
ctrl.addEventListener(CustomEv.HAPPY_END, onHappyEnd);
ctrl.addEventListener(CustomEv.SAD_END, onSadEnd);
ctrl.performControllerMethod();
Controller
public function performControllerMethod():void{
(processOk) ? dispatchEvent(new CustomEv(CustomEv.HAPPY_END, theData)) : dispatchEvent(new CustomEv(CustomEv.SAD_END));
}
When one of the event handlers kicks into action do a cleanup of the event listeners(via event.currentTarget).
As I realize that this might not be a question, but rather a discussion, I would love to get your opinions.
Thanks.
IMO, this whole question is framed in a way that misses the point of MVC, which is to avoid coupling between the model, view, and controller tiers. The View should know nothing of any other tier, because as soon as it starts having references to other parts of the architecture, you can't reuse it.
Static variables that are not constant are basically just asking for trouble http://misko.hevery.com/2009/07/31/how-to-think-about-oo/. Some people believe you can mitigate this by making sure that you only access these globals by using a Controller, but as soon as all Views have a Controller, you have a situation where the static Model can be changed literally from anywhere.
If you want to use the principles of a framework without using a particular framework, check out http://www.developria.com/2010/04/combining-the-timeline-with-oo.html and http://www.developria.com/2010/05/pass-the-eventdispatcher-pleas.html . But keep in mind that established frameworks have solved most of the issues you will encounter, you're probably better off just using one.
I am working on a design spec for a new application that will be heavily workflow driven.
Before I re-invent the wheel, is there a decent lightweight workflow engine that plugs into ASP.NET already around?
Basically, I'm looking for something that handles moving through a defined set of workflow pages while handling state management automatically.
If this isn't around already, I'll definitely try to abstract the engine from my app and put it on codeplex, as it would be really handy.
Any suggestions?
Note: .NET 2.0, so no WWF, though I think WWF is overkill for my needs.
EDIT: Seems like there is a legitimate need for this, and there isn't a product out there...So I might build this.
Here is what I'm picturing:
Custom Page class called WebFlowPage
All WebFlowPage's are registered in a Workflow mapper.
Each WebFlowPage has some form of state object.
A HttpHandler handles picking the appropriate WebFlowPage based upon the workflow, and populating it from the state object.
Is the workflow dynamic, or static?
If the workflows are simple, you could roll your own workflow engine.
In certain situations, it can be fairly simple, and just a couple of data tables to handle the rules, processing and state.
Alot of workflow engines are built for large scale processing (credit card applications, for example). For small scale, you should at least consider your own, which would eliminate the overhead and dependency of/on an engine.
Not sure exactly what you wish to do here, but Ra-Ajax can easily keep state at least if you want your solution ajaxified...
For reference purposes you might want to check out the Ajax Calendar sample or even the (banalistically implemented) Ajax Wizard sample. It surely beats the hell out of doing it with JavaScript...
And every time you "do something" you're in "server-land" which means you can store temporaries all the time as you wish...
The project is LGPL
(PS!
Yes I do work with it)
Building a custom workflow engine is not trivial, although it may seem simple at first. We've tried that. It depends a lot on the complexity of the logic you need it to cover.
Given the current state of the Windows Workflow Foundation and the lack of another framework that abstracts the workflow concepts, I would choose WF if you need complex logic, asynchronous handling or branches in your workflows.
Tracking your state through the workflow can be accomplished by carrying some kind of xml payload or storing the state in a database,
If your workflow is actually a sequential set of forms that need to be filled in by the user, tracking the steps and guiding the user to the next step can be accomplished with some simple custom solution.
You could take a look at the InRule engine too.
Also, there is nxBRE.
These too are mostly used for business rules.
InRule is proprietary, whereas nxBRE supports RuleML (the defacto standard).
You might need to make your own implementation for the pages, and use the rule engine as the "structure".
At this moment, I know that Sharepoint 2007 supports page workflows (using WF), but this would imply using .NET Framework 3 and deployng sharepoint.
My suggestion would be to use whatever you find more light and easier to use.
I think the term "workflow" is very open to interpretation. I have been working lately with a type of workflow that is very different from what you seem to be describing. Mine is a state machine based workflow where the state of a particular record determines what actions a user can take to move the record to the next step in the business process. So "workflow" in this instance means how the record flows from one state to another until it is finally completed.
Your usage of workflow seems to have more to do with moving a user from one page to another in a linear multi-step process, which is a completely different use case (correct me if I'm wrong). So before coming up with a general purpose "workflow" engine that anyone could use, I would recommend defining a little bit better exactly what types of situations this system would handle.
I've been using this for a few months http://objectflow.codeplex.com. Not asp specific but it may fit your needs
While browsing the web for some workflow & BPM resources, I found the following project: NetBPM. Unfortunately, the project seems to be stopped.
I don't think there is a workflow engine that will automatically handle state for you, but if you are moving through a set of pages like a process such as checkout on an ecommerce site, perhaps the ASP.NET wizard control could help you?
There are few workflow options. "Aspose" and "Skelta" are the offers I´m evaluating.
Fábio
you can use WorkFlow Engine, just read the document and run the Demo.
all of the features you need for a dynamic workflow engine they added in there.
I keep hearing about the DRY Principle and how it is so important in ASP.NET MVC, but when I do research on Google I don't seem to quite understand exactly how it applies to MVC.
From what I've read its not really the copy & paste code smell, which I thought it was, but it is more than that.
Can any of you give some insight into how I might use the DRY Principle in my ASP.NET MVC application?
DRY just means "Don't Repeat Yourself". Make sure that when you write code, you only write it one time. If you find yourself writing similar functionality in all of your Controller classes, make a base controller class that has the functionality and then inherit from it, or move the functionality into another class and call it from there instead of repeating it in all the controllers.
use filter attributes to manage aspects (authentication, navigation, breadcrumbs, etc)
use a layer supertype controller (apply common controller-level filters to it, see mvccontrib for an example)
write custom actionresults (like in mvccontrib - for example we made one called logoutresult that just does a FormsAuthentication.Logout()
use a convention for view names
most importantly - keep you controller actions dumb, look for reuse opportunities in services
Don't Repeat Yourself. It can apply to many different aspects of programming. The most basic level of this is prevent code smell. I haven't used ASP.NET so I can't get specific to it and MVC's.
In C++ Templating prevets multiple copies of the same function.
In C void * pointers can be used in a similar fashion, but with great care.
Inheriting from another function allows function allows other functions to use the same code base without having to copy the code.
Normalizing data in a database minimizes redundant data. Also adhereing to the DRY principle.
When you go over a "thought" in a project. Ask yourself.
Have I already wrote this code?
Will this code be useful elsewhere.
Can I save coding by building off of a previous class/function.
DRY is not specific to any one technology. Just make sure you look at your classes from a functionality standpoint (not even from a copy/paste coder view) and see where the duplication occurs. This process will probably not happen in one sitting, and you will only notice duplication after reviewing your code several months later when adding a new feature. If you have unit tests, you should have no fear in removing that duplication.
One advantage of MVC as related to not repeating yourself is that your controller can do tasks common to all pages in the one class. For example, validating against certain types of malicious requests or validating authentication can be centralized.
DRY should not only be applied to code, but to information in general. Are you repeating things in your build system? Do you have data which should be moved to a common configuration file, etc.
Well, the most common example that I can give about DRY and UI is using things like MasterPages and UserControls.
MasterPages ensure that you have written all the static HTML only once.
UserControls ensure reusability of code. Example, you will have a lot of forms doing basic stuff like CRUD. Now, ideally we want all users to see different pages for Create and Update though the forms fields in both will almost be the same. What we can do is combine all the common controls and put them into a control that can be reused over both the pages. This ensures that we are never retyping (or copy-pasting) the same code.
DRY is especially important in MVC because of the increase in the sheer number of files to accomplish the same task.
There seems to be a misconception that everything in a domain model has to be copied up as a special view model. You can have domain models be domain models but view models be something that know nothing of domain specifics and be more generic. For example:
Domain Model classes: Account, Asset, PurchaseOrder
View Model: List, Table, Tuple, SearchFormBackingModel:Checked options, Outputoptions, etc. The view itself might be much more view implementation specific.
The Tuple/Dictonary/Map might map to Account, Asset and PurchaseOrder single instances but a Table might be useful for a collection of them etc. You still have MVC but you have session data, not ready for transaction yet in a view model without necessarily having it violate the rules of your domain model which is where the rules should go. They will be less anemic and anti-pattern that way. You can pass these rules up front and use them there or just in back or both depending on how the system reads from clients etc.