Question

I’m trying to use CF to build a model for an existing database. I have a column in which I forgot to set a sane default value. And rather than compromise the purity of the initial migration by changing it, I just figured I’d create another migration (that’s what migrations are for, right? 🙂

public override void Up()
{
    AlterColumn("Config", "DefaultTaxPerDollar", c => c.Decimal(nullable: false, precision: 19, scale: 5, defaultValue: 0.087m));
}

public override void Down()
{
    AlterColumn("Config", "DefaultTaxPerDollar", c => c.Decimal(nullable: false, precision: 19, scale: 5, defaultValue: 0.0m));

}

But this produces Column already has a DEFAULT bound to it. error from the SQL Server.

How does one change a default value using CF migrations? Or, how does one simply remove a default value (and subsequently re-create it with a different value)?

Edit:

Here is the SQL generated:

ALTER TABLE [Config] ADD CONSTRAINT DF_DefaultTaxPerDollar DEFAULT 0.087 FOR [DefaultTaxPerDollar]
ALTER TABLE [Config] ALTER COLUMN [DefaultTaxPerDollar] [decimal](19, 5) NOT NULL

I think I may have found a solution, to use the Sql() method with some complex SQL inspired by this post. The problem stems from the fact that SQL Server uses constraints to implement defaults (OH! how I miss MySQL!) with a generated name for the constraint. So the Code First team could not simply change or remove/re-create the default value easily.

Answer 1

Removal of default constraints inspired by reverse migrations produced by Entity Framework for SQL Server

    public static void DropDefaultConstraint(string tableName, string columnName, Action<string> executeSQL)
    {
        string constraintVariableName = string.Format("@constraint_{0}", Guid.NewGuid().ToString("N"));

        string sql = string.Format(@"
            DECLARE {0} nvarchar(128)
            SELECT {0} = name
            FROM sys.default_constraints
            WHERE parent_object_id = object_id(N'{1}')
            AND col_name(parent_object_id, parent_column_id) = '{2}';
            IF {0} IS NOT NULL
                EXECUTE('ALTER TABLE {1} DROP CONSTRAINT ' + {0})",
            constraintVariableName,
            tableName,
            columnName);

        executeSQL(sql);
    }

It’s slightly shorter, but the usage is the same.

DropDefaultConstraint(TableName, "DefaultTaxPerDollar", q => Sql(q));

The Guid is used to make a unique variable name in case you are going to drop several constraints in one migration.