EF migrations Code First. Add column to database if column doesn't exist

Question

I need to add a column to a table if that column doesn't exist. Reason: in some database we have such column and in some - not. I've added field to my model and create migration:

public override void Up()
{           
    AddColumn("dbo.NavFilters", "Promo", c => c.String(maxLength: 100, nullable:true));
}

public override void Down()
{
    DropColumn("dbo.NavFilters", "Promo");
}

How can I check is the column exists?

Answer 1

I have worked on creating a custom migration method, AddColumnIfNotExists

You need a custom MigrationOperation class:

public class AddColumnIfNotExistsOperation : MigrationOperation
{
    public readonly string Table;
    public readonly string Name;
    public readonly ColumnModel ColumnModel;

    public AddColumnIfNotExistsOperation(string table, string name, Func<ColumnBuilder, ColumnModel> columnAction, object anonymousArguments) : base(anonymousArguments)
    {
        ArgumentValidator.CheckForEmptyArgument(table, nameof(table));
        ArgumentValidator.CheckForEmptyArgument(name, nameof(name));
        ArgumentValidator.CheckForNullArgument(columnAction, nameof(columnAction));

        Table = table;
        Name = name;

        ColumnModel = columnAction(new ColumnBuilder());
        ColumnModel.Name = name;

    }

    public override bool IsDestructiveChange => false;

    public override MigrationOperation Inverse => new DropColumnOperation(Table, Name, removedAnnotations: ColumnModel.Annotations.ToDictionary(s => s.Key,s => (object)s.Value) , anonymousArguments: null);
}

You also need a custom SqlGenerator class:

public class AddColumnIfNotExistsSqlGenerator : SqlServerMigrationSqlGenerator
{
    protected override void Generate(MigrationOperation migrationOperation)
    {
        var operation = migrationOperation as AddColumnIfNotExistsOperation;
        if (operation == null) return;

        using (var writer = Writer())
        {
            writer.WriteLine("IF NOT EXISTS(SELECT 1 FROM sys.columns");
            writer.WriteLine($"WHERE Name = N'{operation.Name}' AND Object_ID = Object_ID(N'{Name(operation.Table)}'))");
            writer.WriteLine("BEGIN");
            writer.WriteLine("ALTER TABLE ");
            writer.WriteLine(Name(operation.Table));
            writer.Write(" ADD ");

            var column = operation.ColumnModel;
            Generate(column, writer);

            if (column.IsNullable != null
                && !column.IsNullable.Value
                && (column.DefaultValue == null)
                && (string.IsNullOrWhiteSpace(column.DefaultValueSql))
                && !column.IsIdentity
                && !column.IsTimestamp
                && !column.StoreType.EqualsIgnoreCase("rowversion")
                && !column.StoreType.EqualsIgnoreCase("timestamp"))
            {
                writer.Write(" DEFAULT ");

                if (column.Type == PrimitiveTypeKind.DateTime)
                {
                    writer.Write(Generate(DateTime.Parse("1900-01-01 00:00:00", CultureInfo.InvariantCulture)));
                }
                else
                {
                    writer.Write(Generate((dynamic)column.ClrDefaultValue));
                }
            }

            writer.WriteLine("END");



            Statement(writer);
        }
    }
}

And an Extension Method to give you your "AddColumnIfNotExists" function:

public static class MigrationExtensions
{
    public static void AddColumnIfNotExists(this DbMigration migration, string table, string name, Func<ColumnBuilder, ColumnModel> columnAction, object anonymousArguments = null)
    {
        ((IDbMigration)migration)
          .AddOperation(new AddColumnIfNotExistsOperation(table, name, columnAction, anonymousArguments));
    }
}

In your EF Migrations Configuration file, you need to register the custom SQL generator:

[ExcludeFromCodeCoverage]
internal sealed class Configuration : DbMigrationsConfiguration<YourDbContext>
{
    public Configuration()
    {
        AutomaticMigrationsEnabled = false;

        // Register our custom generator
        SetSqlGenerator("System.Data.SqlClient", new AddColumnIfNotExistsSqlGenerator());
    }
}

And then you should be able to use it in place of AddColum like this (notice the this keyword):

[ExcludeFromCodeCoverage]
public partial class AddVersionAndChangeActivity : DbMigration
{
    public override void Up()
    {
        this.AddColumnIfNotExists("dbo.Action", "VersionId", c => c.Guid(nullable: false));
        AlterColumn("dbo.Action", "Activity", c => c.String(nullable: false, maxLength: 8000, unicode: false));
    }

    public override void Down()
    {
        AlterColumn("dbo.Action", "Activity", c => c.String(nullable: false, maxLength: 50));
        DropColumn("dbo.Action", "VersionId");
    }
}

And of course you want some tests for the operation:

[TestClass]
public class AddColumnIfNotExistsOperationTests
{
    [TestMethod]
    public void Can_get_and_set_table_and_column_info()
    {
        Func<ColumnBuilder, ColumnModel> action = c => c.Decimal(name: "T");

        var addColumnOperation = new AddColumnIfNotExistsOperation("T", "C", action, null);

        Assert.AreEqual("T", addColumnOperation.Table);
        Assert.AreEqual("C", addColumnOperation.Name);
    }

    [TestMethod]
    public void Inverse_should_produce_drop_column_operation()
    {
        Func<ColumnBuilder, ColumnModel> action = c => c.Decimal(name: "C", annotations: new Dictionary<string, AnnotationValues> { { "A1", new AnnotationValues(null, "V1") } });

        var addColumnOperation = new AddColumnIfNotExistsOperation("T", "C", action, null);

        var dropColumnOperation = (DropColumnOperation)addColumnOperation.Inverse;

        Assert.AreEqual("C", dropColumnOperation.Name);
        Assert.AreEqual("T", dropColumnOperation.Table);
        Assert.AreEqual("V1", ((AnnotationValues)dropColumnOperation.RemovedAnnotations["A1"]).NewValue);
        Assert.IsNull(((AnnotationValues)dropColumnOperation.RemovedAnnotations["A1"]).OldValue);
    }

    [TestMethod]
    [ExpectedException(typeof(ArgumentNullException))]
    public void Ctor_should_validate_preconditions_tableName()
    {
        Func<ColumnBuilder, ColumnModel> action = c => c.Decimal(name: "T");
        // ReSharper disable once ObjectCreationAsStatement
        new AddColumnIfNotExistsOperation(null, "T", action, null);
    }

    [TestMethod]
    [ExpectedException(typeof(ArgumentNullException))]
    public void Ctor_should_validate_preconditions_columnName()
    {
        Func<ColumnBuilder, ColumnModel> action = c => c.Decimal();
        // ReSharper disable once ObjectCreationAsStatement
        new AddColumnIfNotExistsOperation("T", null, action, null);
    }

    [TestMethod]
    [ExpectedException(typeof(ArgumentNullException))]
    public void Ctor_should_validate_preconditions_columnAction()
    {
        // ReSharper disable once ObjectCreationAsStatement
        new AddColumnIfNotExistsOperation("T", "C", null, null);
    }
}

And tests for the SQL Generator:

[TestClass]
public class AddColumnIfNotExistsSqlGeneratorTests
{
    [TestMethod]
    public void AddColumnIfNotExistsSqlGenerator_Generate_can_output_add_column_statement_for_GUID_and_uses_newid()
    {
        var migrationSqlGenerator = new AddColumnIfNotExistsSqlGenerator();


        Func<ColumnBuilder, ColumnModel> action = c => c.Guid(nullable: false, identity: true, name: "Bar");


        var addColumnOperation = new AddColumnIfNotExistsOperation("Foo", "bar", action, null);

        var sql = string.Join(Environment.NewLine, migrationSqlGenerator.Generate(new[] {addColumnOperation}, "2005")
            .Select(s => s.Sql));


        Assert.IsTrue(sql.Contains("IF NOT EXISTS(SELECT 1 FROM sys.columns"));
        Assert.IsTrue(sql.Contains("WHERE Name = N\'bar\' AND Object_ID = Object_ID(N\'[Foo]\'))"));
        Assert.IsTrue(sql.Contains("BEGIN"));
        Assert.IsTrue(sql.Contains("ALTER TABLE"));
        Assert.IsTrue(sql.Contains("[Foo]"));
        Assert.IsTrue(sql.Contains("ADD [bar] [uniqueidentifier] NOT NULL DEFAULT newsequentialid()END"));
    }
}