EF 4.3 Auto-Migrations with multiple DbContexts in

问题:

I'm trying to use EF 4.3 migrations with multiple code-first DbContexts. My application is separated into several plugins, which possibly have their own DbContext regarding their domain. The application should use one single sql-database.

When I try to auto migrate the contexts in an empty database, this is only successful for the first context. Every other context needs the AutomaticMigrationDataLossAllowed-Property set to true but then tries to drop the tables of the previous one.

So my question is:

• How can I tell the migration-configuration just to look after the tables defined in their corresponding context and leave all others alone?
• What is the right workflow to deal with multiple DbContexts with auto-migration in a single database?

Thank you!

回答1:

Code First Migrations assumes that there is only one migrations configuration per database (and one context per configuration).

I can think of two possible solutions:

1. Create an aggregate context that includes all the entities of each context and reference this "super" context from your migrations configuration class. This way all the tables will be created in the user's database, but data will only be in the ones that they've installed plugins for.

2. Use separate databases for each context. If you have shared entities between the contexts, add a custom migration and replace the CreateTable(...) call with a Sql("CREATE VIEW ...") call to get the data from the entity's "originating" database.

I would try #1 since it keeps everything in a single database. You could create a seperate project in your solution to contain your migrations and this "super" context. Just add the project, reference all of your plugins' projects, create a context that includes all of the entities, then call Enable-Migrations on this new project. Things should work as expected after that.

回答2:

Here is what you can do. very simple.

You can create Configration Class for each of your context. e.g

internal sealed class Configuration1 : DbMigrationsConfiguration<Context1>{
   public Configuration1 (){
        AutomaticMigrationsEnabled = false;
        MigrationsNamespace = "YourProject.Models.ContextNamespace1";
   }
}

internal sealed class Configuration2 : DbMigrationsConfiguration<Context2>{
   public Configuration2 (){
        AutomaticMigrationsEnabled = false;
        MigrationsNamespace = "YourProject.Models.ContextNamespace2";
   }
}

Now you add migration. You dont need to enable migration since you already did with the 2 classed above.

Add-Migration -configuration Configuration1 Context1Init

This will create migration script for context1. your can repeat this again for other Contexts.

Add-Migration -configuration Configuration2 Context2Init

To Update your database

Update-Database -configuration Configuration1
Update-Database -configuration Configuration2

This can be done in any order. Except you need to make sure each configration is called in sequence.

回答3:

I have a working site with multiple contexts using migrations. However, you do need to use a separate database per context, and it's all driven off of a *Configuration class in the Migrations namespace of your project, so for example CompanyDbContext points to Company.sdf using CompanyConfiguration. update-database -configurationtypename CompanyConfiguration. Another LogDbContext points to Log.sdf using LogConfiguration, etc.

Given this works, have you tried creating 2 contexts pointing at the same database and telling the modelbuilder to ignore the other context's list of tables?

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Ignore<OtherContextsClass>();
    // more of these
}

Since the migrations work with the ModelBuilder, this might do the job.

The crappy alternative is to avoid using Automatic Migrations, generate a migration each time and then manually sift through and remove unwanted statements, then run them, although there's nothing stopping you from creating a simple tool that looks at the Contexts and generated statements and does the migration fixups for you.

回答4:

Ok, I have been struggling with this for a day now, and here is solution for those seeking the answer...

I am assuming that most people reading this post are here because they have a large DbContext class with a lot of DbSet<> properties and it takes a long time to load. You probably thought to yourself, gee, that makes sense, I should split up the context, since I won't be using all of the dbsets at once, and I will only load a "Partial" context based on the situation where I need it. So you split them up, only to find out that Code First migrations don't support your way of revolutionary thinking.

So your first step must have been splitting up the contexts, then you added the MigrationConfiguration class for each of the new contexts, you added the connection strings named exactly the same as your new Context classes.

Then you tried running the newly split up contexts one by one, by doing Add-Migration Context1 then doing Update-Database -Verbose...

Everything seemed to work fine, but then you notice that every subsequent Migration deleted all tables from the Previous migration, and only left the tables in from the very last migration.

This is because, the current Migrations model expects Single DbContext per Database, and it has to be a mirror match.

What I also tried, and someone suggested here doing that, is create a single SuperContext, which has All the Db sets in it. Create a single Migration Configuration class and run that in. Leave your partial Context classes in place, and try to Instantiate and use them. The EF complains that the Backing model has changed. Again, this is because the EF compares your partial dbcontext to the All-Sets context signature that was left over from your Super Context migration.

This is a major flaw in my opinion.

In my case, I decided that PERFORMANCE is more important than migrations. So, what I ended up doing, is after I ran in the Super context and had all the tables in place, I went into the database and Manually deleted _MigrationHistory table.

Now, I can instantiate and use my Partial Contexts without EF complaining about it. It doesn't find the MigrationHistory table and just moves on, allowing me to have a "Partial" view of the database.

The trade off of course is that any changes to the model will have to be manually propagated to the database, so be careful.

It worked for me though.

回答5:

As mentioned above by Brice, the most practical solution is to have 1 super DbContext per application/database.

Having to use only 1 DbContext for an entire application seems to be a crucial technical and methodological disadvantage, cause it affects Modularity among other things. Also, if you are using WCF Data Services, you can only use 1 DataService per application since a DataService can map to only 1 DbContext. So this alters the architecture considerably.

On the plus side, a minor advantage is that all database-related migration code is centralized.

回答6:

I just came across this problem and realised the reason I had split them into different contexts was purely to have grouping of related models in manageable chunks and not for any other technical reason. Instead I have declared my context as a partial class and now different code files with different models in them can add DbSets to the DbContext.

This way the automigration magic still works.

回答7:

I've got it working with manual migrations, but you can't downgrade as it can't discrimitate between configurations in the __MigrationHistory table. If I try and downgrade then it treats the migrations from the other configurations as automatic and since I don't allow data loss it fails. We will only ever be using it to upgrade though so it works for our purposes.

It does seem like quite a bit ommision though, I'm sure it wouldn't be hard to support it provided there was no overlap between DbContexts.

回答8:

Surely the solution should be a modification by the EntityFramework team to change the API to support the direct modification of the _MigrationHistory table to a table name of your choice like _MigrationHistory_Context1 such that it can handle the modification of independent DbContext entities. That way they're all treated separately, and its up to the developer to ensure that the names of entities don't collide.

Seems like there are a lot of people who share my opinion that a duplicate DbContext with references to the superset of entities is a bogus non-enterprise friendly way to go about things. Duplicate DbContexts fail miserably for modular (Prism or similar) based solutions.

回答9:

I want people to know that the answer with this below is what worked for me but with one caveat: don't use the MigrationsNamespace line.

internal sealed class Configuration1 : DbMigrationsConfiguration<Context1>{
       public Configuration1 (){
        AutomaticMigrationsEnabled = false;
        MigrationsNamespace = "YourProject.Models.ContextNamespace1";
   }
 }

internal sealed class Configuration2 : DbMigrationsConfiguration<Context2>{
   public Configuration2 (){
        AutomaticMigrationsEnabled = false;
        MigrationsNamespace = "YourProject.Models.ContextNamespace2";
   }
}

However, I already had the 2 databases established with their own contexts defined so I found myself getting an error saying "YourProject.Models namespace already has ContextNamespace1 defined". This was because the "MigrationsNamespace = "YourProject.Models.ContextNamespace2";" was causing the dbcontext to be defined under the YourProjects.Models namespace twice after I tried the Init (once in the migration Context1Init file and once where I had it defined before).

So, I found that what I had to do at that point was start my database and migrations from scratch (thankfully I did not have data I needed to keep) via following the directions here: http://pawel.sawicz.eu/entity-framework-reseting-migrations/

Then I changed the code to NOT include the MigrationsNamespace line.

internal sealed class Configuration1 : DbMigrationsConfiguration<Context1>{
       public Configuration1 (){
        AutomaticMigrationsEnabled = false;
   }
 }

internal sealed class Configuration2 : DbMigrationsConfiguration<Context2>{
   public Configuration2 (){
        AutomaticMigrationsEnabled = false;
   }
}

Then I ran the Add-Migration -configuration Configuration1 Context1Init command again and the Update-Database -configuration Configuration1 line again (for my 2nd context too), and finally, everything seems to be working great now.