I've recently started developing in .NET core.
When developing I encountered the situation that I have to make very similair models that aren't quite the same. For example, let's talk about a booking model:
Frontend: Here I need a model that gets posted as a JSON to my backend and gets deserliazed to a sort of FrontendBooking model.
Backend: I need to add Customer data to the booking, therefore I need to add fields like: CustomerName and CustomerAddress, based on their CustomerId. The backend needs to provide this data, I do not want the frontend to determine these fields. I combine these models to prepare it for an API call. To a model called RequestBooking.
API: I sent RequestBooking to an API and get a response with a similair object that has for example a Status and BookingId added, this was added to the model by the API. So I need to deserialize this to an object called: ResponseBooking.
Database: Finally I wish to store the object to a database, not all properties of the model are relevant however, therefore I create another model called: DatabaseBooking and store this to the databse.
When a property is added, removed or changed. Then I'll have to change it for each of these models.
Is there a design pattern or some other solution, so this is more manageable?
Also, what is best practise for naming these models? Naming them all Booking doesn't feel quite right and adding what they're used for doesn't feel quite right either.
Thanks in advance.
Well, in general you will need different (although similar) models at least at these levels:
Server: here you can make use of Domain Driven Design. You will have an object Booking that is responsible for its logic and contain all properties and methods like e.g. MarkAsCancelled. You can use Entity Framework to use the same object in the database, which will correspond to a database table. EF allows you to mark some properties as not being saved in the DB. Also you can set up EF in the DbContext class and thus not use DB specific attributes in the class. So one object for DB and backend business logic.
API: obviously you cannot send your domain object to the API, e.g. REST. In the API you may want to combine properties of several domain objects or hide some properties. You will have to define a set of Data Transfer Objects (DTOs), e.g. BookingDto. How to convert your domain objects to DTOs? Solutions like AutoMapper may help. You just set up convertion rules once.
Now you can describe your API in e.g. Swagger. With Swagger Codegen you can than generate code for your server (.net) and client (e.g. JS).
In the end you will have to support the following:
API definition (e.g. Swagger). Code for server DTOs and client
objects is autogenerated. You modify API definition once, both sides
get new objects.
DDD Models that also are used for the Database. They
may be faily independent from your DTOs. Mapping is handled for you
semi-automatically by e.g. Automapper
All said is just a suggestion. All the layers and number of objects can and should be adapted to the specific needs of your project. E.g. you may want to use separate objects for the database if you are not using a relational mapper like EF or do not want to mix DB and logic.
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 MVC 3 web app where I get record(s) from DB which is used to render page elements and populate different partial views.
I have classes that represent these DB objects (service layer).
I have also separate set of classes which holds models get returned by the controllers to the view(s).
In my controller, I query DB that returns object to represent DB record.
Then I transfer (MAP) that DB Object to object that Represent the model used by the view(s)
These classes are big and I have to write lots of code in the controller to map.
In most cases I only have some properties which are different.
It seems lots of extra work to do this mapping & lots of code to do the mapping
That is why I am asking.
Is this the correct design approach of developing in MVC framework?
If no, then do you have some pointers that outline the best practice on this aspect.
The Model or ViewModel should only contain information that is used by the View. In some cases, this can be almost identical to what the objects you get from the database are,but this is not always the case. Keeping these concerns separate is good for a number of reasons beyond the scope of a stackoverflow answer. On a side note, I hope you have a separate data access layer and am not querying the database , entity framework or service directly from the controller, again, just to keep those concerns separate.
You can use AutoMapper to map between your DB objects and view models automatically.
Example:
SomeViewModel model = Mapper.Map<SomeViewModel>(someDbObj);
Getting started guide.
I've got to the point with Symfony 2 / Doctrine 2 where I've come to realise that we have build too much business logic in our application into services & controllers - and not enough into the model.
We wish to introduce configuration to our models (to modify behaviour) potentially giving models access to services directly in order to carry out their behaviours.
I've noticed that the following question has the completely wrong answer marked as correct with 8 upvotes - so I know the approach we have taken up to now (anaemic model) is considered the 'correct' way to do things by a lot of Symfony 2 users. After reading more into domain driven design I know this is not the case.
Symfony2 MVC: where does my code belong?
I see a lot of bundles define the behaviour in the model and extend this in entities/documents. This pattern works to a certain extent - but I think we need to introduce an additional stage. Some of the behaviour of our models is optional and having that behaviour will depend on what additional bundles are registered in our application (so including X bundle will allow the application to do more things). An Example.
We have an order object which at the moment has a bidirectional relationship with entities in the courier bundle meaning there is a hard dependency. I want to decouple this and have the courier bundle(s) optionally add behaviour to the order. Consider this method call.
// no courier bundle is registered
$order->getShippingMethods();
// throws NoAvailableShippingMethodsException;
// one bundle registered
$order-getShippingMethods();
// returns an array with one shipping method
etc....
Now currently we have an OrderProvider service which just sits on top of the Entity Manager - so if you call
$orderProvider->GetOrder($id);
You just get the entity returned 'direct' from the database. My question here is what pattens are other people using here? I'm thinking about moving all 'business logic' into a model class that the entity extends, having the service layer pull the entity out (entity being dumb record with properties in the database and getters), and then configure the model using configuration (the configuration being injected into the OrderProvider service), which will modify the behaviour of the model. For the example given I might do something like (within the OrderProvider)..
// trimmed down for example purposes by removing exceptions etc.
public function getOrder($id)
{
$order = $this->orderRepository->findOneById($id);
if ($this->couriers){
$order->addCouriers($couriers);
}
return $order;
}
// this function would be called by the courier bundle automatically using semantic configuration / tags / setter injection
public function addCourier(CourierInterface $courier)
{
$this->couriers[] = $courier;
}
The other option that I have is to create a new type of object - which decorates the base order and is already configured (as it ITSELF will be defined as a service in the DIC) and inject the order into that. The difference is subtle and both approaches would work but I'm wondering which is the best path.
Finally I have one issue with all of this that I can't get my head around. If my base Order entity has relationships with other entities and THOSE entities need to be configured - where should this happen? For example if I access my customer thus.
$order->getCustomer();
I get the customer (entity). But It may be the case that I need to add some configuration to the customer object too - like
$customer->getContactMethods();
Now the behaviour of this method might differ depending on whether my application has registered a twitter bundle or a facebook bundle or something else. Given the above example I'm not going to get a sufficiently configured customer - but rather the 'vanilla' base entity. Only way I can see around this is to cut relationships between entities which require configuration and pull the entity from a CustomerProvider service:
$customerProvider->getCustomerByOrder($order);
This seems to me to be removing information from the model layer and moves back towards a reliance on using multiple services for simple tasks (which I'm trying to get away from). Thoughts and links to resources appreciated.
Edit: Relevant - I see the cons listed in the first answer every single day which is why I've asked this question -> Anemic Domain Model: Pros/Cons
It seems like a complexity of your project is the modularity requirement - application behavior must be extensible via bundles. I'm not familiar with Symfony 2 / Doctrine 2 but a typical DDD tactic is to try and make sure that domain entities such as Order and Customer are unaware of bundle configurations. In other words, surrounding services should not add bundle-specific behaviors to entities. Delegating the responsibility for bundle awareness to entities will make them too complex. Fabricating entity class hierarchies to support extensive behavior is also too complex. Instead, this extensibility should be managed by application services. An application service would determine which bundles are loaded and orchestrate the appropriate entities as a result.
Another strategic pattern to consider is bounded contexts. Is it possible to partition your application into bounded contexts which align with the modules? For example, to address the $order-getShippingMethods() method you can create two BCs, one where there is an order model that has a getShippingMethods() method and another without it. Having two models may seem like a violation of DRY but if the models represent different things (ie an order that has shipping data vs an order that doesn't) then nothing is actually repeated.
Is it possible to call multiple stored procedures (not multiple result sets from a procedure) and display results on a single page in ASP.NET MVC 3 application?
From what I understand only one Model can created on any single page and my stored procedure is already tied to that Model. I would like to call another procedure and display that result as well on my page
I think the root problem is to understand the meaning of the Model in the MVC pattern.
First of all,
The model consists of application data and business rules, and the controller mediates input, converting it to commands for the model or view.[3] A view can be any output representation of data, such as a chart or a diagram
source
In ASP.Net MVC you link a model to your view, this model should not be part of your domain logic or any domain object
The real model (using the meaning of the MVC pattern) is represented by your domain objects.
So what should you put inside the object that you link to your view??
These objects should contain a representation of the view, in other words simple DTO's containing only the data that is going to be used in the view and nothing more. These models should represent the data being used in the view. If you follow this approach, and you need to display more data in the page, you only need to add another property to this model and voila, you can use it from your view.
In a CQRS architecture, these DTO's should be populated by the Query repositories.
If you do not have a CQRS architecture, just populate these objects in your domain, repositories, etc. Do not do it inside the controller, keep your controllers clean and simple, by making calls to your real domain using services or repositories
Try to avoid the reuse of these DTO's, they should belong to one view only. And do yourself a favor and do not try to reuse a domain object instead of a DTO just to use it as the model.
Following this approach your view-models will be clean, since they will be only DTO's and only containing the data needed by the view. And you can fill these DTO's from different sources, even from different databases if you want.
When you want to perform an action you would read from the model the data provided by the user, and with this data you would call your domain through repositories, services or in a CQRS arc. using commands
The simple answer to your question is "yes".
I suggest you do some more research (ie reading articles and looking at sample apps) into MVC and concentrate on understanding these points:
The Model is a class used to group the data you want to display in the View. It can be populated by a variety of methods and does not have to be the domain object or the pure representation of the database result.
A "page" (the concept of what a user sees in their browser window) can be made up from one or more Views. Each View can be responsible for displaying one type of Model allowing for reuse, but a "page" can have multiple Views.
Models are not "tied" to stored procedures. Perhaps you are using an ORM tool that returns a DTO class (which you call model)? This doesn't have to be the Model used by the View. The Controller could compose several of these DTO classes into one Model class.
N-tier application design where database access is separated from the display logic. MVC tries to encourage this but it still has to be done correctly to avoid tying yourself in knots.
Good luck!
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.