Is it a best practice to implement repository pattern with entity framework 6 (and upper versions)? why? It seems that Microsoft doesn't recommend it!
I think it is a good idea to add repository pattern over entity-framework as it can help you alot in many areas. But it can also add a new layer of complexity. So points to consider are:
Using repository you can limit clients to specific operations. (Can be a pro or con depending on requirements and implementation)
You can also provide ready made functions for complex operations so client don't have to repeats that logic.
Repositories can be made thread safe as DbContext isn't.
Repositories will allow you to be independent of entity framework so in future if you ever need to move away from it you can just changed underlying functionality easily.
You can intercept incoming db operations in repository and do whatever you like with them. e.g. add addition where clause in multi-company scenario.
Testing becomes more easier as it becomes easy to mock underlying functionality.
But Repositories also have others cons.
Look at these Is the Repository pattern useful with Entity Framework?
and Benefit of Unit of Work and Repository Pattern with Entity Framework
I am using Entity Framework 6 database-first. I am converting the project to implement the onion architecture to move towards better separation of concerns. I have read many articles and watched many videos but having some issues deciding on my solution structure.
I have 4 projects: Core, Infrastructure, Web & Tests.
From what I've learned, the .edmx file should be placed under my "Infrastructure" folder. However, I have also read about using the Repository and Unit of Work patterns to assist with EF decoupling and using Dependency Injection.
With this being said:
Will I have to create Repository Interfaces under CORE for ALL entities in my model? If so, how would one maintain this on a huge database? I have looked into automapper but found issues with it presenting IEnumererables vs. IQueryables but there is an extension available it has to hlep with this. I can try this route deeper but want to hear back first.
As an alternative, should I leave my edmx in Infrastructure and move the .tt T4 files for my entities to CORE? Does this present any tight coupling or a good solution?
Would a generic Repository interface work well with the suggestion you provide? Or maybe EF6 already resolves the Repository and UoW patterns issue?
Thank you for looking at my question and please present any alternative responses as well.
I found a similar post here that was not answered:
EF6 and Onion architecture - database first and without Repository pattern
Database first doesn't completely rule out Onion architecture (aka Ports and Adapters or Hexagonal Architecture, so you if you see references to those they're the same thing), but it's certainly more difficult. Onion Architecture and the separation of concerns it allows fit very nicely with a domain-driven design (I think you mentioned on twitter you'd already seen some of my videos on this subject on Pluralsight).
You should definitely avoid putting the EDMX in the Core or Web projects - Infrastructure is the right location for that. At that point, with database-first, you're going to have EF entities in Infrastructure. You want your business objects/domain entities to live in Core, though. At that point you basically have two options if you want to continue down this path:
1) Switch from database first to code first (perhaps using a tool) so that you can have POCO entities in Core.
2) Map back and forth between your Infrastructure entities and your Core objects, perhaps using something like AutoMapper. Before EF supported POCO entities this was the approach I followed when using it, and I would write repositories that only dealt with Core objects but internally would map to EF-specific entities.
As to your questions about Repositories and Units of Work, there's been a lot written about this already, on SO and elsewhere. You can certainly use a generic repository implementation to allow for easy CRUD access to a large set of entities, and it sounds like that may be a quick way for you to move forward in your scenario. However, my general recommendation is to avoid generic repositories as your go-to means of accessing your business objects, and instead use Aggregates (see DDD or my DDD course w/Julie Lerman on Pluralsight) with one concrete repository per Aggregate Root. You can separate out complex business entities from CRUD operations, too, and only follow the Aggregate approach where it is warranted. The benefit you get from this approach is that you're constraining how the objects are accessed, and getting similar benefits to a Facade over your (large) set of database entities.
Don't feel like you can only have one dbcontext per application. It sounds like you are evolving this design over time, not starting with a green field application. To that end, you could keep your .edmx file and perhaps a generic repository for CRUD purposes, but then create a new code first dbcontext for a specific set of operations that warrant POCO entities, separation of concerns, increased testability, etc. Over time, you can shift the bulk of the essential code to use this, while still keeping the existing dbcontext so you don't lose and current functionality.
I am using entity framework 6.1 in my DDD project. Code first works out very well if you want to do Onion Architecture.
In my project we have completely isolated Repository from the Domain Model. Application Service is what uses repository to load aggregates from and persist aggregates to the database. Hence, there is no repository interfaces in the domain (core).
Second option of using T4 to generate POCO in a separate assembly is a good idea. Please remember that your domain model (core) should be persistence-ignorant.
While generic repository are good for enforcing aggregate-level operations, I prefer using specific repository more, simply because not every Aggregate is going to need all of those generic repository operations.
http://codingcraft.wordpress.com/
(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.
HI Guys,
I was watching these videos series about Entity Framework:
http://msdn.microsoft.com/en-us/data/ff191186.aspx
is that easy building application in real world programming??? and is it ....reliable...has good performance...
"I am a graduate.."
thanks
Entity Framework is a valid real world data access tool. It is very easy to get up and running with EF. You simply import (or create in EF 4) your data model. You then can rename it to make it more code friendly. And then you are off querying databases.
Performance
I have been on multiple projects that use it, some which require high throughput, others that have low performance requirements. Entity Framework out of the box is not the fastest solution in the world, so there are a lot of performance tweaks that have to go on, but its all do able.
Reliability
We never have issues with reliability. We have never had an issue with EF in general, its always data content related. Trying to insert duplicated data, etc.
Other Tangibles
EF follows a pattern which allows for you to do some fun stuff with templates and abstract classes. All entities inerit from a class, entities that have references inherit from other classes. All Entity Contexts inherit from ;) ObjectContext classes, which provide a base set of functionality that allows you to create generic DAO implementations that can be reused throughout the enterprise.
If you are using UI dev, you can also use Data Services that wrap EF, as a fast gateway to your databse. The only downside of this is that you dont have access to the full suite of the Entity Framework.
I'm developing a program which allows users to input some information which then gets stored and dynamically creates an image based on it.
I was going to use the Entity Framework to do the work with the data, but then I obviously need a way to generate the image. My thinking was that the "correct" way to do this was to somehow extend the data entity to include a function call like "CreateImage", or alternatively, to create a separate class not in the EF called "DataImage" which would have a "generate" method.
Extending the EF seems the "pure" way to do this, but I'm not sure how or if it's more practical than using the separate class.
Any thoughts on the best way to do this and how to do it using EF?
Putting this functionality in the EF would be a major violation of SRP. Breaking SRP has cascading negative effects as your application grows.
The approach you most likely want to take instead is a totally separate, encapsulated image generation service which takes interfaces that your EF entities implement. This decouples your image service from your data access completely; you get complete testability and zero dependencies right away.