外部SQL服务器上的实体框架代码优先

Question

I'm following the Entity Framework tutorial on: Link

I've downloaded the source code, and ran. The project works fine (using the default connection string).

<add name="SchoolContext" connectionString="Data Source=|DataDirectory|School.sdf" providerName="System.Data.SqlServerCe.4.0" />

Next i've changed the connection string to connect to a remote server (which successfully connects). However the tables aren't created and when running the application and accessing the controller I get the following error.

Error:

Model compatibility cannot be checked because the database does not contain 
model metadata. Model compatibility can only be checked for databases created 
using Code First or Code First Migrations.

My database user is 'dbowner' so I wouldn't imagine it's database access issues.

I'm new to EF, and don't know much about Code First Migrations. Have you come across the above error, and would Code Migrations solve this issue? If so why?

Answer 1

From my reading (please correct me if I am wrong) the scenario here is that you have an existing (perhaps empty) database on a remote server that you wish to put your EF code-first work into.

The error is coming about because, I think, EF is looking for a _MigrationHistory table (metadata about what EF code-first migrations have been run against it etc) and can't find it. There is some reasonable coverage of this table here for some background/interest reading.

So to resolve the issue, I think the steps to take are:

1. Initialise your database so that it acknowledges the EF code-first stuff
2. Update the database, rolling forward all your migrations to date

I found some good coverage of how to do this here . This blog also has some good coverage of EF topics in general

PS. I am guessing that your .dsf/SQL Compact Edition database wouldn't have had this problem because EF code-first would have created it for you on first run so it was already acknowledged as being a code-first database.

Answer 2

Here is a link to Entity Framework Power Tools. It is made for creating models by 'Reverse Engineering' your Database on a remote server. You can then easily access this database using EF.

Reverse Engineer Code First - Generates POCO classes, derived DbContext and Code First mapping for an existing database

Answer 3

Both of the initializer methods which I had tried fail when the database already exists:

Database.SetInitializer<Context>(new Initializer());
Database.SetInitializer(new DropCreateDatabaseIfModelChanges<Context>());

However it is possible to force the database to be dropped using:

Database.SetInitializer(new DropCreateDatabaseAlways<Context>());

The following SO post provides the answer to my question: Setting up a Entity Framework Code First Database on SQL Server 2008

I've tried a combination of the two, and have decided that the best solution is to manually go into SQL Management studio and DROP the database, and re-create it using the initializer as this allows me to Seed the contents of the database.

Database.SetInitializer<Context>(new Initializer());

See here for more information on Seeding the database as it is also quite an unstable processess! How can I get my database to seed using Entity Framework CodeFirst?