I'm starting to develop an application, and I want to know the best practices to organize the architecture of the solution.
Should I use EF Class Model as my ViewModel?
Should I put all my queries and db access in the model? or create a Service to manage all Db concerns ?
I'm using EF with DB First, because the db is already developed.
Thanks!
There are much more complete descriptions of application architecture out there, but here's the $.25 description.
EF Class Models are for your communication with a data store
Data Transfer Objects (DTO) are how modules communicate among themselves
(WebAPI to MVC, etc.)
ViewModels supply the data your UI requires
Look up "Separation of Concerns" as it pertains to application architecture, it can save your butt. Often, developers will dual-purpose these entities leading to some hilarious results when you find you've painted a corner for yourself. Not so funny if you are the "painter".
On the other hand, keeping these models requires extra effort and the mappings take CPU cycles. Here's a concrete example:
WebAPI accesses People entity (EF class) and maps to a PeopleDTO (not all fields, maybe additional information) and returns this to your MVC controller. MVC controller takes the PeopleDTO and merges it with supporting lookup tables (more WebAPI calls) to create a PeopleVM (ViewModel) that is used by your Razor Page.
In the scenario I just outlined, there are three different types of People object but each could have very different contents dependent on the needs of that "layer". Lots of tools exist to make the mapping less painful.
Clear?
We are currently rewriting an existing internal ASP.NET Web Forms application. Our application consists of a Web Api back end which uses Entity Framework 6 for data access and an front end which uses AngularJS.
We have an existing large database that I've created EF models using the Code-First Using Existing Database method and we are using data transfer object classes as inputs/outputs to our API methods so we aren't directly exposing our model classes. So basically, I'm trying to become proficient with EF, Web Api and AngularJS all at the same time. For the most part I'm fairly comfortable with the latter two, but for EF I haven't completely gotten comfortable with. I've watched a lot of the videos on Microsoft Virtual Academy but this is the first time I've had some hands-on experience with it.
We've been working on this application for a few months and so far we've only had to work with CRUD operations on our entities (POCO DTO's) which are flat objects with simple properties. However, we've finally come across some situations where we need to deal not only with our classes, but properties which are classes themselves; a parent-child relationship.
Therefore, I have the following questions:
I see that when we have a proper foreign key relationship in our DB, that virtual properties are created in EF, which from what I recall are to support lazy-loading. However, lazy-loading isn't really feasible in this environment where we are using web services (Web Api). Our object model does allow for some really large hierarchy of classes where a fully populated object and its children would mean a large amount of data would be passed around when that really isn't necessary, so in most cases a first level object is all we need. In some cases however, we do want to populate child classes, so my question is how do we do that, and where do we do that? I've looked at the automatically-generated code in the DB Context but we have also used scaffolded code to create our controllers. Which place do we need to do this? I've seen code samples showing how do to this but it hasn't said specifically where this code lies. It appears to be within a controller but I could be wrong.
If we do allow for 2- or more level hierarchy of objects, does EF automatically handle operations (updates, deletes, etc.) -- for example, if we have a "Company" object which has a collection of "Customer" objects, and we delete the "Company" object, do the related "Customer" objects get deleted too? Also, is a multi-step operation like that automatically performed within a transaction or do we need to explicitly set that up?
If I modify a model class or the DB context, seeing as this code is automatically-generated, that's generally bad practice as my changes could be overwritten, so I am assuming the controller code is where I want to make my changes. I am aware of database migrations but I have no experience with them and I am sure I'll need to use them at some point because I am fairly confident that our database may not have all the foreign key relationships necessary for EF to do everything we need at the moment.
I know this is a long post, but if anyone can give some guidance on how to do some of these things because it's not only me that's having to deal with this but I have two other developers on my team who are working on this project and we are all as inexperienced with this as the others are. Thanks
For the purpose of sending data across a web service, I'd suggest creating a DTO to hold the data you want to send and mapping your entities to the DTO instead of trying to send the entities themselves in your payload. It also protects your API from changes to your entity.
Cascading deletes are configurable, iirc, but I'm not 100% sure what the default is. Transactions are generally not implicit, so you will want to use those where you require them.
Not exactly sure what you are asking here. In general, how your entities/tables change depends on if you are using database-first or code-first. If you are using database-first (you will have a .edmx file in your solution that has the model matching your schema), you just update the SQL directly and update your entity model via the .edmx. If you use code-first, you will change the entities how you want them and run a database migration to update your database to match.
MSDN article about code-first migration: https://msdn.microsoft.com/en-us/data/jj591621.aspx
(Note: My question has very similar concerns as the person who asked this question three months ago, but it was never answered.)
I recently started working with MVC3 + Entity Framework and I keep reading that the best practice is to use the repository pattern to centralize access to the DAL. This is also accompanied with explanations that you want to keep the DAL separate from the domain and especially the view layer. But in the examples I've seen the repository is (or appears to be) simply returning DAL entities, i.e. in my case the repository would return EF entities.
So my question is, what good is the repository if it only returns DAL entities? Doesn't this add a layer of complexity that doesn't eliminate the problem of passing DAL entities around between layers? If the repository pattern creates a "single point of entry into the DAL", how is that different from the context object? If the repository provides a mechanism to retrieve and persist DAL objects, how is that different from the context object?
Also, I read in at least one place that the Unit of Work pattern centralizes repository access in order to manage the data context object(s), but I don't grok why this is important either.
I'm 98.8% sure I'm missing something here, but from my readings I didn't see it. Of course I may just not be reading the right sources... :\
I think the term "repository" is commonly thought of in the way the "repository pattern" is described by the book Patterns of Enterprise Application Architecture by Martin Fowler.
A Repository mediates between the domain and data mapping layers,
acting like an in-memory domain object collection. Client objects
construct query specifications declaratively and submit them to
Repository for satisfaction. Objects can be added to and removed from
the Repository, as they can from a simple collection of objects, and
the mapping code encapsulated by the Repository will carry out the
appropriate operations behind the scenes.
On the surface, Entity Framework accomplishes all of this, and can be used as a simple form of a repository. However, there can be more to a repository than simply a data layer abstraction.
According to the book Domain Driven Design by Eric Evans, a repository has these advantages:
They present clients with a simple model for obtaining persistence objects and managing their life cycle
They decouple application and domain design from persistence technology, multiple database strategies, or even multiple data sources
They communicate design decisions about object access
They allow easy substitution of a dummy implementation, for unit testing (typically using an in-memory collection).
The first point roughly equates to the paragraph above, and it's easy to see that Entity Framework itself easily accomplishes it.
Some would argue that EF accomplishes the second point as well. But commonly EF is used simply to turn each database table into an EF entity, and pass it through to UI. It may be abstracting the mechanism of data access, but it's hardly abstracting away the relational data structure behind the scenes.
In simpler applications that mostly data oriented, this might not seem to be an important point. But as the applications' domain rules / business logic become more complex, you may want to be more object oriented. It's not uncommon that the relational structure of the data contains idiosyncrasies that aren't important to the business domain, but are side-effects of the data storage. In such cases, it's not enough to abstract the persistence mechanism but also the nature of the data structure itself. EF alone generally won't help you do that, but a repository layer will.
As for the third advantage, EF will do nothing (from a DDD perspective) to help. Typically DDD uses the repository not just to abstract the mechanism of data persistence, but also to provide constraints around how certain data can be accessed:
We also need no query access for persistent objects that are more
convenient to find by traversal. For example, the address of a person
could be requested from the Person object. And most important, any
object internal to an AGGREGATE is prohibited from access except by
traversal from the root.
In other words, you would not have an 'AddressRepository' just because you have an Address table in your database. If your design chooses to manage how the Address objects are accessed in this way, the PersonRepository is where you would define and enforce the design choice.
Also, a DDD repository would typically be where certain business concepts relating to sets of domain data are encapsulated. An OrderRepository may have a method called OutstandingOrdersForAccount which returns a specific subset of Orders. Or a Customer repository may contain a PreferredCustomerByPostalCode method.
Entity Framework's DataContext classes don't lend themselves well to such functionality without the added repository abstraction layer. They do work well for what DDD calls Specifications, which can be simple boolean expressions sent in to a simple method that will evaluate the data against the expression and return a match.
As for the fourth advantage, while I'm sure there are certain strategies that might let one substitute for the datacontext, wrapping it in a repository makes it dead simple.
Regarding 'Unit of Work', here's what the DDD book has to say:
Leave transaction control to the client. Although the REPOSITORY will insert into and delete from the database, it will ordinarily not
commit anything. It is tempting to commit after saving, for example,
but the client presumably has the context to correctly initiate and
commit units of work. Transaction management will be simpler if the
REPOSITORY keeps its hands off.
Entity Framework's DbContext basically resembles a Repository (and a Unit of Work as well). You don't necessarily have to abstract it away in simple scenarios.
The main advantage of the repository is that your domain can be ignorant and independent of the persistence mechanism. In a layer based architecture, the dependencies point from the UI layer down through the domain (or usually called business logic layer) to the data access layer. This means the UI depends on the BLL, which itself depends on the DAL.
In a more modern architecture (as propagated by domain-driven design and other object-oriented approaches) the domain should have no outward-pointing dependencies. This means the UI, the persistence mechanism and everything else should depend on the domain, and not the other way around.
A repository will then be represented through its interface inside the domain but have its concrete implementation outside the domain, in the persistence module. This way the domain depends only on the abstract interface, not the concrete implementation.
That basically is object-orientation versus procedural programming on an architectural level.
See also the Ports and Adapters a.k.a. Hexagonal Architecture.
Another advantage of the repository is that you can create similar access mechanisms to various data sources. Not only to databases but to cloud-based stores, external APIs, third-party applications, etc.
You're right,in those simple cases the repository is just another name for a DAO and it brings only one value: the fact that you can switch EF to another data access technique. Today you're using MSSQL, tomorrow you'll want a cloud storage. OR using a micro orm instead of EF or switching from MSSQL to MySql.
In all those cases it's good that you use a repository, as the rest of the app won't care about what storage you're using now.
There's also the limited case where you get information from multiple sources (db + file system), a repo will act as the facade, but it's still a another name for a DAO.
A 'real' repository is valid only when you're dealing with domain/business objects, for data centric apps which won't change storage, the ORM alone is enough.
It would be useful in situations where you have multiple data sources, and want to access them using a consistent coding strategy.
For example, you may have multiple EF data models, and some data accessed using traditional ADO.NET with stored procs, and some data accessed using a 3rd party API, and some accessed from an Access database living on a Windows NT4 server sitting under a blanket of dust in your broom closet.
You may not want your business or front-end layers to care about where the data is coming from, so you build a generic repository pattern to access "data", rather than to access "Entity Framework data".
In this scenario, your actual repository implementations will be different from each other, but the code that calls them wouldn't know the difference.
Given your scenario, I would simply opt for a set of interfaces that represent what data structures (your Domain Models) need to be returned from your data layer. Your implementation can then be a mixture of EF, Raw ADO.Net or any other type of Data Store/Provider. The key strategy here is that the implementation is abstracted away from the immediate consumer - your Domain layer. This is useful when you want to unit test your domain objects and, in less common situations - change your data provider / database platform altogether.
You should, if you havent already, consider using an IOC container as they make loose coupling of your solution very easy by way of Dependency Injection. There are many available, personally i prefer Ninject.
The domain layer should encapsulate all of your business logic - the rules and requirements of the problem domain, and can be consumed directly by your MVC3 web application. In certain situations it makes sense to introduce a services layer that sits above the domain layer, but this is not always necessary, and can be overkill for straightforward web applications.
Another thing to consider is that even when you know that you will be working with a single data store it still might make sense to create a repository abstraction. The reason is that there might be a function that your application needs that your ORM du jour either does badly (performance), not at all, or you just don't know how to make the ORM bend to your needs.
If you are wrapping your ORM behind a well thought out repository interface, you can easily switch between different technologies as you see fit. It's not uncommon in my repositories to see some methods use EF for their work and others to use something like PetaPoco, or (gasp) ADO.net code. The repository abstraction enables you to use exactly the right tool for the job at hand without leaking these complexities into the client code.
I think there is a big misunderstanding of what many articles call "repository." And that's why there are doubts about what real value those abstractions bring.
In my opinion the repository in it's pure form is IEnumerable, while you and many articles are talking about "data access service."
I've blogged about it here.
I have an architectural question about EF and WCF.
We are developing a three-tier application using Entity Framework (with an Oracle database), and a GUI based on WPF. The GUI communicates with the server through WCF.
Our data model is quite complex (more than a hundred tables), with lots of relations. We are currently using the default EF code generation template, and we are having a lot of trouble with tracking the state of our entities.
The user interfaces on the client are also fairly complex, sometimes an object graph with more than 50 objects are sent down to a single user interface, with several layers of aggregation between the entities. It is an important goal to be able to easily decide in the BLL layer, which of the objects have been modified on the client, and which objects have been newly created.
What would be the clearest approach to manage entities and entity states between the two layers? Self tracking entities? What are the most common pitfalls in this scenario?
Could those who have used STEs in a real production environment tell their experiences?
STEs are supposed to solve this scenario but they are not silver bullet. I have never used them in real project (I don't like them) but I spent some time playing with them. The main pitfalls I found are:
Coupling your data layer with your client application - you must share entity assembly between projects (it also means it is .NET only solution but it should not be a problem in your case)
Large data transfers - you pass 50 entities to clients, client change single entity and you will pass 50 entities back. It will require some fighting with STEs to avoid passing unnecessary data
Unnecessary updates to database - normally when EF works with attached entities it track changes on property level but with STEs it track changes on entity level. So if user modify single property in entity with 100 properties it will generate update with setting all of them. It will require modifying template and adding property level change tracking to avoid this.
Client application should use STEs directly (binding STEs to UI) to get most of its self tracking ability. Otherwise you will have to implement code which will move data from UI back to self tracking entity and modify its state.
They are not proxied = they don't support lazy loading (in case of WCF service it is good behavior)
I described today the way to solve this without STEs. There is also related question about tracking over web services (check #Richard's answer and provided links).
We have developed a layered application with STE's. A user interface layer with ASP.NET and ModelViewPresenter, a business layer, a WCF service layer and the data layer with Entity Framework.
When I first read about STE's the documentation said that they are easier then using custom DTO's. They should be the 'quick and easy way' and that only on really big projects you should use hand written DTO's.
But we've run in a lot of problems using STE's. One of the main problems is that if your entities come from multiple service calls (for example in a master detail view) and so from different contexts you will run into problems when composing the graphs on the server and trying to save them. So our server function still have to check manually which data has changed and then recompose the object graph on the server. A lot has been written about this topic but it's still not easy to fix.
Another problem we ran into was that the STE's wouldn't work without WCF. The change tracking is activated when the entities are serialized. We've originally designed an architecture where WCF could be disabled and the service calls would just be in process (this was a requirement for our unit tests, which would run a lot faster without wcf and be easier to setup). It turned out that STE's are not the right choice for this.
I've also noticed that developers sometimes included a lot of data in their query and just send it to the client instead of really thinking about which data they needed.
After this project we've decided to use custom DTO's with automapper from server to client and use the POCO template in our data layer in a new project.
So since you already state that your project is big I would opt for custom DTO's and service functions that are a specifically created for one goal instead of 'Update(Person person)' functions that send a lot of data
Hope this helps :)
Hi
I've been building the backend of my application (using nHibernate for data access).
So far i've had some simple web-services for manipulating my data, but now I'm required to develop a web UI on for my application (using web forms).
I've been looking at some different web frameworks (webformsMVP, spring.net web), and also at some client-side JS frameworks (knockout, angular), and I can't really decide what would be the best for me, and how to integrate it all.
I was hoping to get some insight from you guys.
I think I would like for my general workflow to be something like this:
View is created, calls its presenter.
Presenter contacts business layer to retrieve information (which in turn contacts DAO etc.)
Presenter returns a view-model object, which the view displays.
User manipulates data (maybe using some AJAX to retrieve further needed information)
View sends a view-model of the manipulated data back to presenter
Presenter translates the view-model into a model entity and sends it to business layer
now here are the parts that I find tricky:
A. How to map between view-model and model entities
I think I should use the entitie's ID to retrieve the unchanged entity from the DAO (which actually stores it in nHibernate's 2nd level cache), and perform changes on it.
(another option is to store the entity i'm currently editing in the user's session. but i'm afraid that this kind of caching would create duplicity with nHibernate's cache.)
B. How to translate changes on the view-model into changes on model entities
I'd like to have some logic when changing model entity's properties.
For example, for moving an employee from one department to another, I don't want to allow this-
department1.Employees.Remove(employee);
department2.Employees.Add(employee);
but rather this:
employee.MoveToDepartment(department2);
I'm afraid this could get complicated when translating from a view-model into a model.
Any thoughts on the above two questions, and also about any client-side / server side frameworks would be appriciated.
P.S. some quick background on my app:
-one page of the web app displays the company's structure (departments, divisions etc.) as a tree, and allows the user to click and edit the different nodes, as well as drag-and-drop nodes to change their location.
-another page displays current stock status (for each warehouse- how many products it has, how many machines are currently operative in that warehouse etc.)
- (some more pages which basically display data and allow editing...)
thanks
Jhonny
You can check out Automapper for your entity mapping. Alternatively you can write your own entity mappers and entity updaters. You can then encapsulate the business logic like Move To Department in your entity classes.
Your approach sounds just fine, and I can recommend Webforms.MVP as that is a good framework for writing testable WebForms apps without rolling your own implementation.