Let's say I have a set of classes that I want to share across multiple projects. For instance, I could use them in a REST service and also in a client that consumes that service.
So I create the following projects:
MyOrders.Models
MyOrders.RestApi
MyOrders.Client
Both the RestApi and Client projects have dependencies on the Models project.
The RestApi is using Entity Framework (code first) so normally you'd decorate the model's properties with things like [NotMapped] and [Key]. However, I don't want the Client solution to have any dependency on Entity Framework. None. So I can't decorate the models' properties with EF-specific attributes.
So my question is, is there some way to correctly set the models' EF-specific attributes from the RestApi project instead, maybe in the Context's constructor or something?
You can have the POCOs in your Models project, keep them totally ignorant of Entity Framework, and do the mappings in a separate project or in the RestApi project itself.
You can do this by the fluent mapping API, for instance in the OnModelCreating override of the context that you create in the EF-aware project:
modelBuilder.Entity<Order>().HasKey(o => o.OrderID);
modelBuilder.Entity<Order>().Ignore(o => o.OrderTotal);
etc.
This is a good argument for using custom Data Transfer Objects that are independent of the table-like entities. Although it can feel like overkill to have nearly duplicate classes - one as DTOs and one as EF Entities - there is another long-range benefit: the two sets of classes can vary independently. Let's say that you change the table table structure, but the client doesn't need to know about this change. Update the EF Entity but you leave the DTO alone, though you may have to update how you map from EF to DTO.
Speaking of mapping: EmitMapper can be a great help in transferring between the two types of objects.
You need to split your data access models from the rest of the application using Data Transfer Objects.
This will give a lot of benefits. At first it will look if your duplicating all the code of the model. But when your application grows, you will find that need the data in a view which is formatted in another way than how it was or is stored the database. Validation attributes can be added in a very specific way just the way you need it.
Mapping in between them can be done various ways. By hand or by using a tool like AutoMapper
Related
I'm working on a new project and wanted to use MVC and entity framework. For the purpose of separation of concerns. I plan to structure my project like so..
MyProject.Web (this project houses the V and C of MVC)
MyProject.Model (this project houses the M of MVC so that it can be reused and shared. This is where my business objects/ domain objects live)
MyProject.BLL (this is where I write my business logic and make available via an interface)
MyProject.Entity (this is my DAL - where entity objects will be generated by the wizard using db first approach)
My question is what is the best way to convert entity object to business object in my BLL? My mapping requires that I join two tables and compute a sum that would map to a field in a business object.
Not sure this is a good example but let say I join customer table and order table and got two records back for the same customer. One is for order placed in the AM and one for order placed in the PM. I need to compute the total for the day and mapped it to a field in my business object.
Automapper comes to mind instead of manually code DTO, but I am not sure if it can do complex mapping (with the sum calculation)?
Is there a way to manually configure custom map in EF 6 to do this?
With Automapper, you can use Projections for complex mappings or use AutoMapper's QueryableExtensions helper methods.
See Aggregations section in the following link.
LINQ can support aggregate queries, and AutoMapper supports LINQ
extension methods
https://github.com/AutoMapper/AutoMapper/wiki/Queryable-Extensions
Let's say I have a project where I use Entity Framework, but I want to use my own classes instead of the EF classes.
Reasons for using my own classes:
Easy to add properties in code
Easy to derive and inherit
Less binding to the database
Now, my database has table names like User and Conference.
However, In my domain project, I also call my files User.cs and Conference.cs.
That means I suddenly have two objects with the same naming, which is usually very annoying to work with, because you have to use namespaces all the time to know the difference.
My question is how to solve this problem?
My ideas:
Prefix all database tables with 'db'. I usually do this, but in this case, I cannot change the database
Prefix or postfix all C# classes with "Poco" or something similar
I just don't like any of my ideas.
How do you usually do this?
It's difficult to tell without more background but it sounds like you are using the Entity Framework designer to generate EF classes. This is known as the "Model First" workflow. Have you considered using the Code First / Code Only workflow? When doing code first you can have POCO classes that have no knowledge of the database, EF, or data annotations. The mapping between the database and your POCOs can be done externally in the the DBContext or in EntityTypeConfiguration classes.
You should be able to achieve your goal of decoupling from EF with just one set of objects via code first.
To extend the above answer, the database table name User (or Users as many DB designers prefer) is the identifier for the persistence store for the object User that's defined in your code file User.cs. None of these identifiers share the same space, so there should be no confusion. Indeed, they are named similarly to create a loose coupling across spaces (data store, code, development environment) so you can maintain sanity and others can read your code.
I've been reading articles on StackOverflow and other sites all day about best architecture practices and there are just so many conflicting ideas and opinions.
I've finally settled on an approach, but I am having a really hard time deciding where to place the EF objects (DbContext, Fluent APIs, Seeding data, etc). Here is what I currently have:
ASP.NET MVC Project: The actual web project. Contains the standard views, controllers and View Models (inside a Models folder).
Domain Model Project: Contains all POCO classes that define the database (domain) objects. Currently, does not mention or reference any EF objects.
Service Layer Project: Contains service objects for each type of domain object (e.g., IProductService, IOrderService, etc). Each service references EF objects like DbSets and handles business rules - e.g., add a Product, fetch a Product, append a Product to an Order, etc.
So the question is, in this configuration, where do EF classes go? Initially I thought in the Service Layer, but that doesn't seem to make sense. I then thought to put them in the Domain Model Layer, but then it ties the Domain Models to EF, which is essentially a DAL / Repository. Finally, I thought about creating a separate DAL Project just for EF, but it seems like a huge waste considering it will likely have 3-4 files in it (DbContext and a few other small files).
Can anyone provide any guidance?
There is no need for Domain Model since it will be redundancy. EF classes directly can act as Domain Model and they are converted to View Models while sending it to View. EF can be separated into different class library. Most of them use repository pattern along with any ORM incase it would be easy if they go for replacement. But I've seen criticism over using repository pattern, check this out.
Here is what I do:
Data:
Has one class inheriting from DbContext.
It has all the db sets.
Overrides OnModelCreating.
Mapping primary keys and relationships.
Entities:
Has every POCO classes.
Each property is decorated with needed data annotations.
Services:
Each service has common methods (GetList(), Find(), Create(), etc.).
Business:
Called from clients, orchestrate using services to perform a specific task UserChangePassword (this will check if this can be performed, then perform the task, or return error/unauthorized statuses among many others to make the client shows the correct information regarding the task. This on my case is where I log.
Clients (Desktop/Web/Wpf/etc).
I'm not saying this is the best approach, I'm just sharing what's been working for me.
Please help on choosing the right way to use the entities in n-tier web application.
At the present moment I have the following assembleis in it:
The Model (Custom entities) describes the fields of the classes that the application use.
The Validation is validating the data integrity from UI using the reflection attributes method (checks data in all layers).
The BusinessLogicLayer is a business facade for additional logic and caching that use abstract data providers from DataAccessLayer.
The DataAccessLayer overrides the abstarct data providers using LinqtoSql data context and Linq queries. And here is the point that makes me feel i go wrong...
My DataLayer right before it sends data to the business layer, maps (converts) the data retrieved from DB to the Model classes (Custom entities) using the mappers. It looks like this:
internal static model.City ToModel(this City city)
{
if (city == null)
{
return null;
}
return new model.City
{
Id = city.CountryId,
CountryId = city.CountryId,
AddedDate = city.AddedDate,
AddedBy = city.AddedBy,
Title = city.Title
};
}
So the mapper maps data object to the describing model. Is that right and common way to work with entities or do I have to use the data object as entities (to gain a time)? Am I clear enough?
You could use your data entities in your project if they are POCOs. Otherwise I would create separate models as you have done. But do keep them in a separate assembly (not in the DataAccess project)
But I would not expose them through a webservice.
Other suggestions
imho people overuse layers. Most applications do not need a lot of layers. My current client had a architecture like yours for all their applications. The problem was that only the data access layer and the presentation layer had logic in them, all other layers just took data from the lower layer, transformed it, and sent it to the layer above.
The first thing I did was to tell them to scrap all layers and instead use something like this (requires a IoC container):
Core (Contains business rules and dataaccess through an orm)
Specification (Seperated interface pattern. Contains service interfaces and models)
User interface (might be a webservice, winforms, webapp)
That works for most application. If you find that Core grows and becomes too large too handle you can split it up without affecting any of the user interfaces.
You are already using an ORM and have you thought about using a validation block (FluentValidation or DataAnnotations) for validation? Makes it easy to validate your models in all layers.
It may be a common practice to send out DTOs from serivce boundary (WCF service, etc.) but if you are directly using your "entities" in your presentation model, I don't see any benefit in doing that.
As to the code snippet you have provided, why not use AutoMappter? It helps by eliminating writing of boiler-plate mapping codes and does that for you if you have a set of convention in place.
Get rid of the model now, before removing it later will require refactoring the whole application. The last project i worked on used this architecture and maintaining the DTO layer and mappings to the database model layer is a huge pain in the arse and offers no usefull benefits. One of the main things that is anoying is that LinkToSql does not effectively support a disconnected data model. You cannot update a database table by creating a new DB entity with a primary key matching an existing record and then stick it into the data context. You have to first retrieve the entity from the database, update it then commit the changes. Managing this results in really nasty update methods to map all the properties from your DTOs to your LinqtoSql classes. It also breaks the whole deferred execution model of LinqToSql. Don't even get me started on the problems it causes with properties on parent classes that are collections of child DTOs (e.g. a customer DTO with an Orders property that contains a collection of order DTOs), managing those mappings is really really fiddly, i had to do some extensive optimisations because retrieving a few hundred records ended up causing LinqToSql to make 200,000 database calls (admittedly there was also some pretty dumbass code as well but you get the picture).
The only valid reason to use DTOs is if you want to have multiple pluggable Data Access Layers e.g. LinqToSql and NHibernate for supporting different DB servers. That way you can swap out the data access later without having to change any other layers. If you don't need to do this then save yourself a world of pain and just use the LinqToSql entities.
I am working on a project where we need to fetch data from a WCF service. The service code looks up the database thru the Entity Framework. However inorder to prevent sending down EF generated classes across the wire into the proxy generated by the client we have decided to map the values from the EF classes to custom built DTO classes, where the mapper class is responsible for picking out values from the EF generated classes and putting them into the DTO class. We then use those DTO classes for the service method's request and response.
The EF builds classes from tables that are related to each other. I get various classes with properties that look something like these below:
public global::System.Data.Objects.DataClasses.EntityCollection<SubAttachment> Attachments
{}
public global::System.Data.Objects.DataClasses.EntityReference<Gl> GlCodeReference
{}
A few of the properties have the keyword Reference appended to them which I am guessing provides some way for the EF to look up the related table on that field.
Is there a better/different approach than this mapping approach if I dont want to send heavy EF classes across? If not, is there some reference material that will help me understand how the the classes are built by the Entity framework.
Thanks for your time
Since you need to fetch data from WCF service which is backed by EF framework, have you considered using OData to expose EF objects? Check out some links below:
http://www.odata.org/
http://www.hanselman.com/blog/ODataBasicsAtTheAZGroupsDayOfNETWithScottGu.aspx
Link
When you create classes in EF, they have the [DataMember] attributes on their fields, and that's the only data that get's sent accross the wire. So, it's not as heavy as it seems...
But, since you're passing through WCF, the entities should be generated to be self-tracking, so when they get back to the service, you know what's changed and don't have to refetch every entity from db to do comparing.
If you still want the DTO's, you can generate them as well. If you're using EF4.0 you have an option of extracting a T4 file (.tt) that practically does the code generation - use that and alter to suit your needs and generate DTO's as well as mapper classes...
To get a .tt file from edmx (only for EF4): right click your model, choose Add code generation items, and choose EntityObject generator, or the other one if you want to have objects transfered through wcf. This will create a tt file that you can run by issuing a save command (you'll get a prompt if you want to allow it to run). When saved, it will generate a file that's exactly the same as the file generated by edmx model in the case of EntityObject generator, or you'll have two .tt files if you're using the other generator...
I've used something very similar to the approach in the link below along with some custom partial classes and it worked quite nicely.
Link