AddPostCommandHandler.cs

public class AddPostCommandHandler : IRequestHandler<AddPostCommand, string>
{
    public string Handle(AddPostCommand command)
    {
        var param = new
        {
            PublishDate = DateTime.Now,
            Slug = SlugConverter.Convert(command.Title),
            command.Title,
            command.Body,
            command.Published
        };

        using (var connection = SqlConnectionFactory.Create())
        {
            if (SlugTaken(connection, param.Slug))
            {
                return null;
            }

            connection.Execute(
                "INSERT INTO [Posts] VALUES (@Slug, @Title, @Body, @Published, @PublishDate)",
                param);

            return param.Slug;
        }
    }

    private static bool SlugTaken(IDbConnection connection, string slug)
    {
        var param = new { Slug = slug };
        var record = connection.ExecuteScalar(
            "SELECT TOP 1 [Slug] FROM [Posts] WHERE [Slug] = @Slug", 
            param);

        return record != null;
    }
}