Thursday, July 19, 2012

[C# and EF] Tutorial: Entity Framework Code First Migrations 1/2

I showed you how to use the Entity Framework Code First approach in one of my last blog posts. This approach is very useful if you want to begin with the implementation and there is no database yet. You want to concentrate on the code and not worry about the database at all, therefore letting Entity Framework auto-generate the database for you.

This is especially interesting for developers not having too much experience in database development or in cases where you have to deliver very quickly and you just don’t want to bother with database design.

The problem : Lack of handling Code First model changes in the DB

Code First works very well and is very easy to use. But it lacks an important feature as I have already pointed out in one of my other blog posts – the handling of database schema changes (add/delete/modify columns, add/delete/modify data types, add/delete/modify constraints, etc…) related to EF Code First model changes .

When you change the underlying EF Code First model expressed in your code (via POCO classes for example) and/or its restrictions you either have to re-create the whole database, meaning that all your data gets lost in the process (not acceptable in production use), or code the database changes manually via SQL upgrade scripts.

In this case the developer still needs to have good database development skills and spent time to create, test and execute the upgrade scripts. This is a tedious task, which the new version of Entity Framework 5.0 integrated in .NET 4.5  is going to greatly simplify and automate.

The solution: Code First Migrations

Microsoft heard the customer requests concerning this problem and added what is called Entity Framework Migrations. It is very easy to activate, and though not perfect, serves very well for applying necessary database schema changes if you work with EF Code First.

Let me show you how it works by providing an example in Visual Studio 2012.

  • Create a new project (for the example a console project but you may as well use any other type of project).

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  • Now do a right click on the project in the Solution Explorer and select to manage the NuGet packages for it.

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  • In the Nuget packages window search for Entity Framework and select version 5.0 (currently as a release candidate, this won’t be necessary in the future since it will be fully integrated in .NET 4.5).

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  • This will allow you to use Code First and do an example implementation : a POCO class Person.
  • When you run your application and use the DataContext, the database and all its columns and constraints get automatically generated. Everything works fine and your application works as expected.

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  • Now you get an application change request and you decide to change the EF Code First model by changing the POCO class.

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  • But if you try to change the underlying POCO classes of an already generated database and execute your application, you will get the following exception.

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  • This is the expected behavior since the new EF Code First Migrations features are not activated by default. You have to activate them manually and configure them according to your needs.
  • To do this open the Package Manager Console from the Tools menu within Visual Studio 2012 and activate the EF Code First Migration features from there.

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In the next part of the series I am going to explain how to activate either the manual or the automatic features of EF Code First Migrations so stay tuned.


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1 comment:

Annie Calvert said...

Nice post and very helpful tutorial ,more tutorial you can read from dapfor. com