DTO/POCO with Entity Framework

Question

I'm using EF5 Model First. I don't really understand what are the auto-generated classes from the EDM. According to some documentation this classes are POCOs but why are they used in the context ? Assuming I have a Student entity, then I get a Student POCO class and a DbSet StudentSet property in my context. Will this next instructions put a POCO in my database ?

MyContext.StudentSet.Add(johndoe);
MyContext.SaveChanges();

So EF uses POCO to transfer data ? Actually I miss the step when POCO exchange data with entities or DTO and when the entities put data in the database.

What have you tried, yes, your code will put JohnDoe in your database. If you carry data on DTO, then you need set POCO based on the properties from DTO. — J.W., Sep 13 '14 at 13:28

Yuliam Chandra · Answer 1 · 2014-09-13T15:26:10.353

The generated classes from the EDM is the ORM / Persistence classes. You use that classes to query / make changes from / to database. You need to translate any DTO object to POCO object when about making changes to database.

ORM is about mapping object to data in database, instead of dealing with insert into syntax to insert record to database in the application, you use StudentSet.Add to add a new data. The johndoe information will be translated into sql syntax, EF will map each property to each column when translating it into query.

The Add method will store the johndoe information as Added in the memory but it will not be executed right away to the database. If you have another Add method, it will be marked as Added too. The moment you call SaveChanges, all the changes will be saved into database by sending a generated query.

The mapping between DTO and EF entity happens before you add the johndoe. You might have another DTO class that is used in the UI. You need to map it manually or using mapper library to create a POCO object from a DTO object. For example:

// studentDto as parameter

var johndoe = new Student
{
    Name = studentDto.StudentName,
    Age = studentDto.StudentAge
};
MyContext.StudentSet.Add(johndoe);

// studentDto might have another information as well
var johndoeSubject = new Subject
{
    Name = studentDto.SubjectName,
    Years = studentDto.SubjectYears
};
MyContext.SubjectSet.Add(johndoeSubject);

MyContext.SaveChanges();

Thanks Yuliam but why you add a Student and not a StudentDto to your context ? — Llm, Sep 13 '14 at 21:45
@Llm, because the table Student in the database is mapped into Student class, not StudentDto class, you can open the EDMX file with xml editor and see the mapping confguration that is used by EF, the CSDL, SSDL and MSL — Yuliam Chandra, Sep 14 '14 at 03:36
Ok I get it now. So I should use DTO to pass data to my views ? Actually I was searching for a way to add a layer between what I get from EF and what I pass to my views to add some security... — Llm, Sep 15 '14 at 08:01
@Llm, I'm not suggesting to create the DTO for the view as there have been other posts that describing this, like [this one](http://stackoverflow.com/questions/725348/poco-vs-dto) and [this one](http://stackoverflow.com/questions/18109547/orm-entities-vs-domain-entities-under-entity-framework-6-0), but if you have DTO entity and want to make changes to the database, you need to convert the DTO entity to ORM Entity. Do you need more help other than that? — Yuliam Chandra, Sep 15 '14 at 08:45

DTO/POCO with Entity Framework

1 Answers1