Changing column default values in EF5 Code First

问题:

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.

回答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.

回答2:

Here's a solution that was inspired by this post. It's not exactly an elegant method, but it works for me.

        public static void DropDefaultConstraint(string tableName, string columnName, Action executeSQL)
        {
            // Execute query that drops the UDF that finds the default constraint name
            var query = @"
                    -- recreate UDF 
                    if object_id('[dbo].[GetDefaultConstraintName]') is not null
                    begin 
                        drop function [dbo].[GetDefaultConstraintName]
                    end
                ";
            executeSQL(query);

            // Execute query that (re)creates UDF that finds the default constraint name
            query = @"
                    create function [dbo].[GetDefaultConstraintName] (
                        @TableName varchar(max),
                        @ColumnName varchar(max))
                    returns varchar(max)
                    as
                    begin
                        -- Returns the name of the default constraint for a column

                        declare @Command varchar(max)
                        select
                            @Command = d.name
                        from
                            ((
                            sys.tables t join
                            sys.default_constraints d
                                on
                                    d.parent_object_id = t.object_id) join
                            sys.columns c
                                on
                                    c.object_id = t.object_id and
                                    c.column_id = d.parent_column_id)
                        where
                            t.name = @TableName and
                            c.name = @ColumnName
                        return @Command
                    end
                ";
            executeSQL(query);

            // Execute query that actually drops the constraint
            query = string.Format(@"
                    -- Use UDF to find constraint name
                    DECLARE @Constraint_Name VARCHAR(100)
                    SET @Constraint_Name = [dbo].GetDefaultConstraintName('{0}','{1}')

                    if LEN(@Constraint_Name) > 0 
                    BEGIN
                        DECLARE @query VARCHAR(300)
                        SET @query = 'ALTER TABLE {0} DROP CONSTRAINT ' + @Constraint_Name

                        execute(@query)
                    END", tableName, columnName);
            executeSQL(query);
        }

And in your migration, you can call it like this:

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

The reason for using the lamba is because you have to make three distinct calls to Sql(). I was never able to get this to work as one long query - tried many combinations of the keyword GO in many different places. I also tried reversing the logic on the first query so that the UDF only gets recreated if it does not exist, and it didn't work. I suppose recreating it every time is more robust anyway.