实体框架 - 代码优先方法

Question

I am just following the entity framework code first approach in my project, and I fell into some problems. Always I am forced to regenerate the db using the commands. however, now I am managed to get that stable. I have a scenario and in this, the framework is creating a migration script is not logical, may be I am missing something and need your help in that

The scenario is - I have enabled the migration, and It created an InitialCreate class with all my table setup, I am pretty impressed with that. but when i make change in the entity, for eg, I have Deal Class

I added new property called LenderName, BorrowerName, and run teh add-migration script, it created a migration script for me ,

 public partial class LenderBorrowerName : DbMigration
    {
        public override void Up()
        {
            DropColumn("dbo.Deals", "LenderName");
            DropColumn("dbo.Deals", "BorrowerName");
            DropColumn("dbo.Deals", "Discriminator");
        }

        public override void Down()
        {
            AddColumn("dbo.Deals", "Discriminator", c => c.String(nullable: false, maxLength: 128));
            AddColumn("dbo.Deals", "BorrowerName", c => c.String());
            AddColumn("dbo.Deals", "LenderName", c => c.String());
        }
    }

The problem is, the original database doesn't have this field and through this migration script it suppose to add to the database. To update these fields, if I run the update-migration -force, it fails because of the up() function above, as it try to remove the column from the table. Why Microsoft is adding that drop script, is that the developer need to remove manually ? please help

Answer 1

The hint here is the Discriminator column. EF applies a default strategy to implement inheritance which is called TPH, Table-Per-Hierarchy. The Discriminator field is automatically generated by EF when you have inheritance between your entities.

It seems that you did more changes in your model than adding those two properties to an existing entity: you probably moved them from a derived class to a base class, or the opposite way around. Also it seems that your DbContext only includes a DbSet for the base entity or the derived entity, but not for both. Be aware that migrations by default only include the entities that are included in your DbContext as DbSets (you can also include or exclude them explicitly in your mappings). So your migration is generating only the fields for the entity in your DbContext , and not for the other one, and EF is interpreting that you removed those properties, that is why you get DropColumn sentences.

So the solution here would be to add the missing DbSet to the DbContext and generate the migration again.

Here you can have more info about TPH.

A different explanation could be that after running Update-Database and Add-Migration several times and using the -force parameter your database has been left in an state that is inconsistent with the contents of your __MigrationHistory table. My advice there would be to manually delete the database, if possible, generate a clean Initial migration and run it.