Temporal SqlServer Migrations Sql Generator

// Rather than augmenting existing SQL it might make more sense to create new operations.
// E.g. MakeTemporal after CreateTable and DropTemporal before DropTable. Not sure how to cause that to happen for now.
public class TemporalSqlServerMigrationsSqlGenerator : SqlServerMigrationsSqlGenerator
{
    // HACK: I gave up. This is required to explicitly name history tables and I dont know how to get the target schema from this code.
    private const string DefaultSchema = "dbo";

    public TemporalSqlServerMigrationsSqlGenerator(MigrationsSqlGeneratorDependencies dependencies, IMigrationsAnnotationProvider migrationsAnnotations)
        : base(dependencies, migrationsAnnotations) { }

    protected override void ColumnDefinition(AddColumnOperation operation, IModel model, MigrationCommandListBuilder builder)
    {
        DisallowNullablePeriodColumns(operation);
        base.ColumnDefinition(operation, model, builder);
    }

    protected override void Generate(AlterColumnOperation operation, IModel model, MigrationCommandListBuilder builder)
    {
        DisallowNullablePeriodColumns(operation);
        base.Generate(operation, model, builder);
    }

    protected override void Generate(DropColumnOperation operation, IModel model, MigrationCommandListBuilder builder)
    {
        var temporalAnnotation = operation[TemporalAnnotationNames.TemporalComponent] as TemporalComponentType?;
        var isPeriodStart = temporalAnnotation == TemporalComponentType.PeriodStart;
        var isPeriodEnd = temporalAnnotation == TemporalComponentType.PeriodEnd;
        if (isPeriodStart || isPeriodEnd)
        {
            // Period columns cant be dropped
            throw new InvalidOperationException("Period Columns cannot be dropped");
        }

        base.Generate(operation, model, builder);
    }

    protected override void Generate(CreateTableOperation operation, IModel model, MigrationCommandListBuilder builder)
    {
        base.Generate(operation, model, builder);

        var temporalAnnotation = operation[TemporalAnnotationNames.TemporalComponent] as TemporalComponentType?;
        var isTemporal = temporalAnnotation == TemporalComponentType.Table;
        if (isTemporal)
        {
            var tableName = Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Name, operation.Schema ?? DefaultSchema);
            var periodStartColumn = operation.Columns.Single(c => (c[TemporalAnnotationNames.TemporalComponent] as TemporalComponentType?) == TemporalComponentType.PeriodStart);
            var periodEndColumn = operation.Columns.Single(c => (c[TemporalAnnotationNames.TemporalComponent] as TemporalComponentType?) == TemporalComponentType.PeriodEnd);
            var fqPeriodStartColumnName = Dependencies.SqlGenerationHelper.DelimitIdentifier(periodStartColumn.Name);
            var fqPeriodEndColumnName = Dependencies.SqlGenerationHelper.DelimitIdentifier(periodEndColumn.Name);

            builder
                .Append($"ALTER TABLE {tableName} ADD PERIOD FOR SYSTEM_TIME ({fqPeriodStartColumnName}, {fqPeriodEndColumnName})")
                .AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator);

            //TODO: Decide whether we care to make period columns hidden
            builder
                .Append($"ALTER TABLE {tableName} ALTER COLUMN {fqPeriodStartColumnName} ADD HIDDEN")
                .AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator)
                .Append($"ALTER TABLE {tableName} ALTER COLUMN {fqPeriodEndColumnName} ADD HIDDEN")
                .AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator);

            //TODO: Do we want to allow custom table suffixes? Use generated history table name?
            var historyTableName = Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Name + "_History", operation.Schema ?? DefaultSchema);

            builder
                .Append($"ALTER TABLE {tableName} SET (SYSTEM_VERSIONING = ON (HISTORY_TABLE = {historyTableName}))")
                .AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator);

            EndStatement(builder);
        }
    }

    protected override void Generate(DropTableOperation operation, IModel model, MigrationCommandListBuilder builder)
    {
        // Cannot drop a System-versioned table so we have to disable it first
        var temporalAnnotation = operation[TemporalAnnotationNames.TemporalComponent] as TemporalComponentType?;
        var isTemporal = temporalAnnotation == TemporalComponentType.Table;
        if (isTemporal)
        {
            var fullTableName = Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Name, operation.Schema ?? DefaultSchema);

            builder
                .Append($"ALTER TABLE {fullTableName} SET (SYSTEM_VERSIONING = OFF)")
                .AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator);

            builder.AppendLine();
        }

        base.Generate(operation, model, builder);
    }

    private void DisallowNullablePeriodColumns(ColumnOperation operation)
    {
        var temporalAnnotation = operation[TemporalAnnotationNames.TemporalComponent] as TemporalComponentType?;
        var isPeriodStart = temporalAnnotation == TemporalComponentType.PeriodStart;
        var isPeriodEnd = temporalAnnotation == TemporalComponentType.PeriodEnd;
        if (isPeriodStart || isPeriodEnd)
        {
            // Period columns cant be nullable
            if (operation.IsNullable)
            {
                throw new InvalidOperationException("Period Columns cannot be nullable");
            }
        }
    }
}