使用 ALinq 支持多种数据库

１、打开设计器，生成实体类，然后在 External Mapping 属性中，选择 true。Connection 属性中的 Application Setting 设为 true， 然后保存重新生成代码，你会发现多了一个后缀名为 .designer.map 的 Xml 文件 。

生成 Xml 映射文件（局部）

２、在代码中使用 XmlMapping

        public Database()
            : base(WebConfigurationManager.ConnectionStrings["ALinqBBS"].ConnectionString, CreateMappingSource())
        {

        }

        public Database(string connn)
            : base(connn, CreateMappingSource())
        {

        }

        static XmlMappingSource CreateMappingSource()
        {
            var xmlMapping = XmlMappingSource.FromStream(typeof(Database).Assembly.GetManifestResourceStream("ALinq.BBS.Business.Database.designer.map"));
            return xmlMapping;
        }

３、下面我们接着来把数据库转换成 MS SQL 数据库

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            var stream = typeof(Database).Assembly.GetManifestResourceStream("ALinq.BBS.Business.Database.designer.map");
            Debug.Assert(stream != null);
            var reader = new System.IO.StreamReader(stream);

            //将Provider 切换为 Sql2005Provider
            var xmlDatabase = XmlMappingSourceUtility.CreateFrom<Database>(reader);
            xmlDatabase.Provider = "ALinq.SqlClient.Sql2005Provider, ALinq, Version=2.3.0.0, Culture=neutral, PublicKeyToken=2b23f34316d38f3a";
            
            //修改 XmlMapping 中 SQLite 的数据类型为 SQL2005 的数据类型。如果你需要导入数据，并且含还有自增长列，你还得设 DbGenerate 为 false 。
            var xmlColumn = xmlDatabase.Type(o => o.Posts).Column(o => o.Content);
            xmlColumn.DbType = "Text";
            
            xmlColumn = xmlDatabase.Type(o => o.Replies).Column(o => o.Content);
            xmlColumn.DbType = "Text";

            //下面的是 SQL2005 数据库
            var conn = @"Data Source=VPC1\SQLEXPRESS;Initial Catalog=ALinqBBS;User ID=sa;Password=test";
            var db = new Database(conn, XmlMappingSource.FromXml(xmlDatabase.Element.ToString())) { Log = Console.Out };

            var sqlLiteDb = new Database(@"C:\ALinqBBS.db3", XmlMappingSource.FromStream(typeof(Database).Assembly.GetManifestResourceStream("ALinq.BBS.Business.Database.designer.map"))) { Log = Console.Out };
            
            //如查你不需要导入数据库，直接生成数据库就可以了。代码如下：
            //if (db.DatabaseExists())
            //    db.DeleteDatabase();
            //db.CreateDatabase();

            //如查你需要导入数据库，请参考下面的代码。
            var tables = db.Mapping.GetTables();
            foreach (var table in tables)
            {

                //在数据库中创建表
                if (db.TableExists(table))
                    db.DeleteTable(table);

                db.CreateTable(table);
            }

            //导入数据库 开始
            foreach (var board in sqlLiteDb.Boards.ToList())
                db.Boards.Insert(board);

            foreach (var post in sqlLiteDb.Posts.ToList())
                db.Posts.Insert(post);

            foreach (var reply in sqlLiteDb.Replies.ToList())
                db.Replies.Insert(reply);

            foreach (var user in sqlLiteDb.Users.ToList())
            {
                var date = DateTime.Parse("1900-1-1");
                if (user.CreationDate < date)
                    user.CreationDate = date;

                db.Users.Insert(user);
            }

            foreach (var topPost in sqlLiteDb.TopPosts.ToList())
                db.TopPosts.Insert(topPost);

            foreach (var item in sqlLiteDb.PostFavors)
                db.PostFavors.Insert(item);

            foreach (var item in sqlLiteDb.ObjectIdentities)
                db.ObjectIdentities.Insert(item);

            //导入数据库 结束

            //创建关联
            foreach (var table in tables)
            {
                db.CreateForeignKeys(table);
            }
            
        }
    }

    class XMappingDatabase<T>
    {
        public XMappingDatabase(XElement element)
        {
            this.Element = element;
        }

        public XElement Element { get; private set; }

        public string Provider
        {
            get
            {
                var attribute = Element.Attribute("Provider");
                if (attribute == null)
                    return null;
                return attribute.Value;
            }
            set
            {
                var attribute = Element.Attribute("Provider");
                if (attribute == null)
                    Element.SetAttributeValue("Provider", value);
                else
                    attribute.Value = value;
            }
        }
    }

    class XMappingType<T>
    {
        public XMappingType(XElement element)
        {
            this.Element = element;
        }

        public XElement Element { get; private set; }
    }

    class XMappingColumn
    {
        public XMappingColumn(XElement element)
        {
            this.Element = element;
        }

        public XElement Element { get; private set; }

        public string DbType
        {
            get
            {
                var attribute = Element.Attribute("DbType");
                if (attribute == null)
                    return null;
                return attribute.Value;
            }
            set
            {
                var attribute = Element.Attribute("DbType");
                if (attribute == null)
                    Element.SetAttributeValue("DbType", value);
                else
                    attribute.Value = value;
            }
        }
    }

    static class XmlMappingSourceUtility
    {
        public static XMappingType<TEntity> Type<TSource, TEntity>(this XMappingDatabase<TSource> source, Expression<Func<TSource, Table<TEntity>>> predicate) where TEntity : class
        {
            var name = typeof(TEntity).FullName;

            var element = source.Element.Descendants().Where(o => o.Name.LocalName == "Type")
                                                      .Where(o => o.Attribute("Name") != null && o.Attribute("Name").Value == name).SingleOrDefault();
            if (element == null)
                throw new Exception(string.Format("Count Not Found the '{0}' Element in the XDocument", name));

            return new XMappingType<TEntity>(element);
        }

        public static XMappingColumn Column<TSource>(this XMappingType<TSource> source, Expression<Func<TSource, object>> predicate)
        {
            var parameters = predicate.Parameters;
            var name = ((MemberExpression)predicate.Body).Member.Name;

            var element = source.Element.Descendants().Where(o => o.Name.LocalName == "Column")
                                                        .Where(o => o.Attribute("Name") != null && o.Attribute("Name").Value == name).SingleOrDefault();
            if (element == null)
                throw new Exception(string.Format("Count Not Found the '{0}' Element in the XDocument", name));

            return new XMappingColumn(element);
        }

        public static XMappingDatabase<TSource> CreateFrom<TSource>(StreamReader reader)
        {
            var doc = XElement.Load(reader, LoadOptions.None);
            return new XMappingDatabase<TSource>(doc);
        }

    }

}