Error: throw new UnintentionalCodeFirstException()

Code generated using the T4 templates for Database First and Model First development may not work correctly if used in Code First mode. To continue using Database First or Model First ensure that the Entity Framework connection string is specified in the config file of executing application. To use these classes, that were generated from Database First or Model First, with Code First add any additional configuration using attributes or the DbModelBuilder API and then remove the code that throws this exception.

If the connection string has the metadata, EF thinks it is Model First or Database First. If it is a plain connection string, EF thinks it is Code First. However, if you want to start out doing model first but make EF think you are really doing code first (which is what you are doing), make sure you are using the DbContext code generator, not the default one. Code first POCOs are really that--"plain old c# objects"-- no special database aware or change tracking stuff in them at all. To use the DbContext code generator, right click on your model diagram and choose "Add new code generation item..." then select the ADO.NET DbContext Generator. Also, depending on how you named your primary and foreign keys and/or whether they are more complicated than just simple int IDs, you will probably need to fill in some code to map the relationships between your objects in the "OnModelCreating" method in your context. Delete the line throw new UnintendedCodeFirstException(); and replace it with your mapping code. Otherwise EF may not be able to figure out all the relationships (remember there's no metadata for it to rely on).

In my case: I's working on a test project and my database was Northwind DB. I had 3 layer in my project Main project, DAL, BLL. In my executing project connection string was:

<add name="NorthwindDBEntities" providerName="System.Data.SqlClient" connectionString="data source=(local);initial catalog=NorthwindDB;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework" />

So, encountered error!!!
And then, change it to following:

<add name="NorthwindDBEntities" connectionString="metadata=res://*/Northwind.csdl|res://*/Northwind.ssdl|res://*/Northwind.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=(local);initial catalog=NorthwindDB;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />

Look, here I've included the metadata information with along connection string.
Hope this will help :)


Credit goest to:
http://stackoverflow.com/users/10263/brian-rogers