Entity Framework 4.1 code first where to validate

Question

I'm new to Entity Framework, have read Julie Lerman book and lots of articles about it. Have new project that has both database and classes already defined, so have decided to use Code First approach (although it is the new project so database and classes are pretty similar).

So, we have: - database layer with entities, mapping and DbContext - classes (business layer) - WPF with MVVM (UI layer)

If I understand Code First properly, database layer references business layer, UI references both database and business layer. (If I try to add in the business layer reference to database layer, I get circular reference error.)

Basic validation, like required field or length I understand, but where to put additional (more complex) validations if business layer is not aware of database layer?

Thanks, Claire

Answer 1

Database layer doesn't reference business layer and presentation layer doesn't reference database layer. That would break whole meaning of layered architecture. Correct layering is:

Database Layer -> Busienss Layer -> Presentation Layer

What probably confuse you are entities. In simple architecture entities are shared among all layers. To achieve that you must place them to separate assembly used by all layers.

Validation can take place in any layer:

• Presentation layer can validate user input - it can use methods from business layer for that
• Business layer should enforce business rules, it can also expose methods for UI to do user input validation
• Database layer can validate if entities conform to constraints defined in the database (for example are required columns are filled)

Answer 2

you can do complex validations in your entity classes by implementing IValidatableObject interface. Then you can do the validations inside

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        //your validation logic. if there are no errors return an empty list
        return new System.Collections.Generic.List<ValidationResult>();
    }

EF 4.1 is aware of this interface. So it call this method before it saves the changes. If there are any validation errors it will abort the trasnsaction.

Answer 3

Have a look at this blog to get more info about your structure.

http://weblogs.asp.net/scottgu/archive/2010/07/16/code-first-development-with-entity-framework-4.aspx

That should help.

There are a couple more parts to the blog which are worth working through,

http://weblogs.asp.net/scottgu/archive/2010/07/23/entity-framework-4-code-first-custom-database-schema-mapping.aspx

http://weblogs.asp.net/scottgu/archive/2010/08/03/using-ef-code-first-with-an-existing-database.aspx

In general you have options of putting validation in a number of places. Your view models is one place to do it. Throw a ValidationException for instance. If you are using third party controls like telerik then they automatically pick up and display that you have a validation issue.