Entity Framework telling me the model backing the context has changed - entity-framework

Question

I have a weird problem with Entity Framework code first migrations. I've been using EF and code first migrations on a project for months now and things are working fine. I recently created a new migration and when running Update-Database a restored backup of my database I get this error:
The model backing the context has changed since the database was
created. Consider using Code First Migrations to update the database
The migration does something like the following:
public override void Up()
{
using (SomeDbContext ctx = new SomeDbContext())
{
//loop through table and update rows
foreach (SomeTable table in ctx.SomeTables)
table.SomeField = DoSomeCalculation(table.SomeField);
ctx.SaveChanges();
}
}
I'm not using the Sql() function because DoSomeCalculation must be done in C# code.
Usually when I get something like this is means that I have updated my model somehow and forgot to create a migration. However that's not the case this time. The weird thing is that the error isn't even occurring on a migration that I created a few days ago and had been working fine.
I looked a quite a few articles about this and they all seems to say call
Database.SetInitializer<MyContext>(null);
Doing that does seem to work, but my understanding (based on this article) is that doing that will remove EF's ability to determine when the database and model are out of sync. I don't want to do that. I just want to know why it thinks they are out of sync all of a sudden.
I also tried running Add-Migration just to see if what it thought changed about the model but it won't let me do that stating that I have pending migrations to run. Nice catch 22, Microsoft.
Any guesses as to what's going on here?
I'm wondering if maybe the fact that migration listed above is using EntityFramework is the problem. Seems like maybe since it's not the latest migration anymore, when EF gets to it tries to create a SomeDbContext object it checks the database (which is not fully up to date yet since we're in the middle of running migrations) against my current code model and then throws the "context has changed" error.

Answer 1

It's possibly related to your using EF within the migration. I'm not sure how you're actually managing this, unless you've set a null database initialiser.
If you need to update data within a migration, use the Sql function, e.g.
Sql("UPDATE SomeTable SET SomeField = 'Blah'");
You should note that the Up() method is not actually running at the time of doing the migration, it's simply used to set up the migration which is then run later. So although you may think you've done something in the migration above the bit where you're using EF, in reality that won't have actually run yet.
If you cannot refactor your calculation code so it can be written in SQL, then you would need to use some mechanism other than migrations to run this change. One possibility would be to use the Seed method in your configuration, but you would need to be aware that this does not keep track of whether the change has been run or not. For example...
internal sealed class Configuration : DbMigrationsConfiguration<MyContext>
{
public Configuration()
{
AutomaticMigrationsEnabled = false;
}
protected override void Seed(MyContext context)
{
// Code here runs any time ANY migration is performed...
}
}

Answer 2

I tried replacing the EntityFramework code with regular ADO.NET code and it seems to work. Here is what it looks like:
public override void Up()
{
Dictionary<long, string> idToNewVal = new Dictionary<long, string>();
using (SqlConnection conn = new SqlConnection("..."))
{
conn.Open();
using (SqlCommand cmd = new SqlCommand("SELECT SomeID, SomeField FROM SomeTable", conn))
{
SqlDataReader reader = cmd.ExecuteReader();
//loop through all fields, calculating the new value and storing it with the row ID
while (reader.Read())
{
long id = Convert.ToInt64(reader["SomeID"]);
string initialValue = Convert.ToString(reader["SomeField"]);
idToNewVal[id] = DoSomeCalculation(initialValue);
}
}
}
//update each row with the new value
foreach (long id in idToNewVal.Keys)
{
string newVal = idToNewVal[id];
Sql(string.Format("UPDATE SomeTable SET SomeField = '{0}' WHERE SomeID = {1}", newVal, id));
}
}