adding a foreign key with Code First migration

Question

I have an error when trying to update my database after adding a migration.

Here are my classes before add-migration

public class Product
{
    publi         


        

Answer 1

The quick answer is - firstly add a nullable column for ProductId, then set some default value in all existing rows, then set the column to non null (if you need that) for future inserts, add finally add the foreign key constraint.

I wrote a long blog post on just this with full source code included - http://nodogmablog.bryanhogan.net/2015/04/entity-framework-non-null-foreign-key-migration/

Answer 2

I came across this problem today. Here's how I handled it. I did have to make my FK property nullable which was not a big deal in my case.

I made my changes to my POCO, generated the migration and then added the following to the migration.

public partial class LineItemProductionHistory_v4 : DbMigration
{
    public override void Up()
    {
        AddColumn("LineItemProductionHistories","TempLineItemId",c=>c.Int());
        Sql("Update LineItemProductionHistories Set TempLineItemId = LineItem_Id");
       .
       . Generated code
       .
        Sql("Update LineItemProductionHistories Set LineItemId = TempLineItemId");
        DropColumn("LineItemProductionHistories", "TempLineItemId");
    }
}

I'm just temporarily storing all off the foreign keys, letting the migration do it's add/drop stuff and then putting the foreign keys back in the newly created FK.

Answer 3

I'm not sure if this topic is still actual, but at least with EF 6 you can do the following to assign default value into a new foreign key.

This is how EF create a new foreign key by default:

            AddColumn("dbo.Table", "AnotherTableId", c => .Int(nullable: false));
            CreateIndex("dbo.Table", "AnotherTableId");
            AddForeignKey("dbo.Table", "AnotherTableId", "dbo.AnotherTable", "Id");

Just change it as below

            AddColumn("dbo.Table", "AnotherTableId", c => 
            { 
                var model = c.Int(nullable: false);
                model.DefaultValue = 1;
                return model; 
            });
            CreateIndex("dbo.Table", "AnotherTableId");
            AddForeignKey("dbo.Table", "AnotherTableId", "dbo.AnotherTable", "Id");

Answer 4

This is an old issue but currently there is no need to create a separate migration and this issue can be solved using a few steps:

1. Run Add-Migration with the entity changes (a new non-nullable reference property added, ProductId in this case) to scaffold a new migration class
2. Modify newly added migration to create a nullable column (nullable: true) instead of non-nullable
3. Below AddColumn add Sql command setting the value of the column as needed
4. Below add AlterColumn command which will make the column non-nullable as intended

In the example above this would look like:

    public override void Up()
    {
        AddColumn("dbo.ProductFeatures", "ProductId", c => c.Int(nullable: true));
        Sql("UPDATE [dbo].[ProductFeatures] SET ProductId = (SELECT TOP 1 [Id] FROM [dbo].[Products])");
        AlterColumn("dbo.ProductFeatures", "ProductId", c => c.Int(nullable: false));
        CreateIndex("dbo.ProductFeatures", "ProductId");
        AddForeignKey("dbo.ProductFeatures", "ProductId", "dbo.Products", "Id");
    }

Answer 5

It can occur when you try to add foreign key constraint on a non nullable column of a table that already contains data. If your tables contain data try to delete them first and retry to update your database.

Answer 6

The key to solving this problem is to break your migration into two migrations. First, add a nullable field and fill in the data. Second, make the field a required foreign key.

First Migration

1. Add the new property to your class as a nullable type (e.g. int?)

   public class MyOtherEntity
   {
       public int Id { get; set; }
   }
   
   public class MyEntity
   {
       ...
       // New reference to MyOtherEntity
       public int? MyOtherEntityId { get; set; }
       ...
   }
   

2. Create a migration. NOTE: Migration name is not important, but something like "AddBlogPosts1" makes it easy to read.

   > add-migration AddMyEntityMyOtherEntity1
   

3. This should scaffold a migration that looks like this:

   public partial class AddMyTableNewProperty1 : DbMigration
   {
       public override void Up()
       {
           AddColumn("dbo.MyEntity", "MyOtherEntityId", c => c.Int());
       }
       public override void Down()
       {
           DropColumn("dbo.MyEntity", "MyOtherEntityId");
       }
   }
   

4. Now manually edit the generated migration to add a default value for the new field. The easiest case is when the default value is invariant. You can add more logic in the SQL if needed. This example assumed all the MyEntity instances point to the same MyOtherEntity instance with ID 1.

   public partial class AddMyTableNewProperty1 : DbMigration
   {
       public override void Up()
       {
           AddColumn("dbo.MyEntity", "MyOtherEntityId", c => c.Int());
   
           // ADD THIS BY HAND
           Sql(@"UPDATE dbo.MyEntity SET MyOtherEntityId = 1
                 where MyOtherEntity IS NULL");
       }
       public override void Down()
       {
           DropColumn("dbo.MyEntity", "MyOtherEntityId");
       }
   }
   

5. Update the database

   > update-database
   

Second Migration

1. Go back to your MyEntity class and change the new property to represent a mandatory foreign key.

   public class MyEntity
   {
       ...
       // Change the int? to int to make it mandatory
       public int MyOtherEntityId { get; set; }
   
       // Create a reference to the other entity
       public virtual MyOtherEntity MyOtherEntity { get; set; }
       ...
   }
   

2. Create another migration

   > add-migration AddMyEntityMyOtherEntity2
   

3. This should create a migration like the following:

   public partial class AddMyEntityMyOtherEntity2: DbMigration
   {
       public override void Up()
       {
           AlterColumn("dbo.MyEntity", "MyOtherEntityId", c => c.Int(nullable: false));
           CreateIndex("dbo.MyEntity", "MyOtherEntityId");
           AddForeignKey("dbo.MyEntity", "MyOtherEntityId", "dbo.MyOtherEntity", "Id");
       }
       public override void Down()
       {
           DropForeignKey("dbo.MyEntity", "MyOtherEntityId", "dbo.MyOtherEntity");
           DropIndex("dbo.MyEntity", new[] { "MyOtherEntityId" });
           AlterColumn("dbo.MyEntity", "MyOtherEntityId", c => c.Int());
       }
   }
   

4. Update the database

   > update-database
   

Other Notes

1. This technique works for migrations applied during application start up.
2. Adding more complex mappings for the new column in the SQL are possible, but not illustrated here.