昨天（星期五）下班，19：00左右回到家，洗个澡，然后20：30左右开始写代码，写完代码之后，上床看了《生活大爆炸10季》17、18两集，发现没有更新到19集，瞄了一眼手机，竟然已经是凌晨02：00多了，关掉电视睡觉，10：30左右被老婆电话吵醒，洗漱完毕，去麦当劳吃了一个早餐，然后屁颠屁颠地坐地铁到很远的地方去爬山。爬山回来之后，闲来无事，写篇文章记录一下昨晚所花的几个小时干的事情——使用EntityFrameworkCore实现Repository<TEntity>, UnitOfWork<TContext>，支持MySQL分库分表。
由于是使用业余时间写来玩的，时间也有限，所以，全部代码做了一个基本假设：Repository<TEntity>, UnitOfWork<TContext>只支持同一个IP上的MySQL分库分表，不同IP上的MySQL分库分表，需要使用不同的Repository<TEntity>, UnitOfWork<TContext>对象。以下示例代码，假设数据库是按年分库按月分表。
EntityFrameworkCore默认并不支持分库分表，我们看一眼EntityFrameworkCore默认生成的SQL:
Executed DbCommand [Parameters=[@p2='?', @p4='?' (Size = 8000), @p6='?' (Size = 8000)], CommandType='Text', CommandTimeout='0']INSERT INTO `t_user_201703` (`Fis_deleted`, `Fpassword`, `Fname`)VALUES (@p2, @p4, @p6);SELECT LAST_INSERT_ID();默认生成的SQL并没有带上库名，而想要让EntityFrameworkCore支持MySQL分库分表，首要条件是必须能做到可以动态地改变库名表名。软件界有一句老话叫：凡是做不到的就多抽象一层，所以，想要让EntityFrameworkCore支持MySQL分库分表，我抽象了以下两个接口， IRepository<TEntity>和IUnitOfWork
很多人都自己动手实现过Repository和UnitOfWork，虽然各自实现不尽相同，但是其实现本身并没有难度，但在这里，我们需要特别关注两个方法：void ChangeTable(string table)和void ChangeDatabase(string database)
/// <summary> /// Changes the table name. This require the tables in the same database. /// </summary> /// <param name="table"></param> /// <remarks> /// This only been used for supporting multiple tables in the same model. This require the tables in the same database. /// </remarks>
void ChangeTable(string table);/// <summary> /// Changes the database name. This require the databases in the same machine. /// </summary> /// <param name="database">The database name.</param> /// <remarks> /// This only been used for supporting multiple databases in the same model. This require the databases in the same machine. /// </remarks>
void ChangeDatabase(string database);怎么实现这两个方法，就需要一定的技术功底了，我以前在一家创业公司的时候，因为看不惯架构师自以为是的样子，自己动手写了一个轻量级的ORM框架，如果以后有时间，我打算写一篇《如何基于Dapper实现一个轻量级的ORM框架》的文章。ORM框架背后的动机很单纯，就是数据库与Domain之间的一种双向映射，真正把这种单纯的动机搞复杂是的那些性能优化，各种缓存实现。而从Domain到数据库这一单方向上的映射，在.NET领域借助了一种代码即数据的思想，再细化到C#语言代码即数据就是表达式树。所以，我们有理由相信：SQL是根据表达式树生成的。现在我们已经找准了方向，那么我们看看EntityFrameworkCore在什么地方生成表名的，也就是说，我们只需要修改一下生成表名的代码，就可以做到动态生成database.table SQL。EntityFrameworkCore是通过TableExpression来生成表名的：
public class TableExpression{public virtual string Table { get; }public virtual string Schema { get; }}如果你MySQL知识至少跟我一样的水平的话，看到TableExpression表达式有一个Schema是不是立即就可以想到：哈哈，太好了，我压根就不用修改EntityFrameworkCore本身的代码就可以实现。为什么呢？好吧，看看MySQL官网怎么说Schema的:
In MySQL, physically, a schema is synonymous with a database. You can substitute the keyword SCHEMA instead of DATABASE in MySQL SQL syntax, for example using CREATE SCHEMA instead of CREATE DATABASE. Some other database products draw a distinction. For example, in the Oracle Database product, a schema represents only a part of a database: the tables and other objects owned by a single user.
好吧，Schema就是Database，那么我们就用Schema.Table来表示database.table。现在事情就变得简单了，变成了我们如何动态地改变Schema和Table了，以下是我提供的简化实现：
/// <summary> /// Changes the database name. This require the databases in the same machine. /// </summary> /// <param name="database">The database name.</param> /// <remarks> /// This only been used for supporting multiple databases in the same model. This require the databases in the same machine. /// </remarks>public void ChangeDatabase(string database){if (_context.Model.Relational() is RelationalModelAnnotations relational){relational.DatabaseName = database;}var connection = _context.Database.GetDbConnection();if (connection.State.HasFlag(ConnectionState.Open)){connection.ChangeDatabase(database);}var items = _context.Model.GetEntityTypes();foreach (var item in items){if (item.Relational() is RelationalEntityTypeAnnotations extensions){extensions.Schema = database;}}}/// <summary> /// Changes the table name. This require the tables in the same database. /// </summary> /// <param name="table"></param> /// <remarks> /// This only been used for supporting multiple tables in the same model. This require the tables in the same database. /// </remarks>public void ChangeTable(string table){if (_dbContext.Model.FindEntityType(typeof(TEntity)).Relational() is RelationalEntityTypeAnnotations relational){relational.TableName = table;}}OK, 虽然有点low，但是毕竟支持了MySQL分库分表，看看怎么用：
namespace QuickStart.Controllers{[Route("api/[controller]")]public class UserController : ApiController{private readonly IUnitOfWork _unitOfWork;// 1. IRepositoryFactory used for readonly scenario; // 2. IUnitOfWork used for read/write scenario; // 3. IUnitOfWork<TContext> used for multiple databases scenario;public UserController(IUnitOfWork unitOfWork){_unitOfWork = unitOfWork;unitOfWork.ChangeDatabase($"rigofunc_{DateTime.Now.Year}");var userRepo = unitOfWork.GetRepository<User>();var postRepo = unitOfWork.GetRepository<Post>();var ym = DateTime.Now.ToString("yyyyMM");userRepo.ChangeTable($"t_user_{ym}");postRepo.ChangeTable($"t_post_{ym}");var user = new User{//UserId = 123, UserName = "rigofunc",Password = "password"};userRepo.Insert(user);var post = new Post{//PostId = 123, UserId = user.UserId,Content = "What a piece of junk!"};postRepo.Insert(post);unitOfWork.SaveChanges();var find = userRepo.Find(user.UserId);find.Password = "p@ssword";unitOfWork.SaveChanges();}[HttpGet]public IPagedList<User> Get(){_unitOfWork.ChangeDatabase($"rigofunc_2018");var userRepo = _unitOfWork.GetRepository<User>();return userRepo.Query(u => true).OrderBy(u => u.UserId).ToPagedList(0, 20);}}}以下是生成的SQL：
Executed DbCommand [Parameters=[@p2='?', @p4='?' (Size = 8000), @p6='?' (Size = 8000)], CommandType='Text', CommandTimeout='0']INSERT INTO `rigofunc_2017`.`t_user_201703` (`Fis_deleted`, `Fpassword`, `Fname`)VALUES (@p2, @p4, @p6);SELECT LAST_INSERT_ID();Executed DbCommand [Parameters=[@p10='?' (Size = 8000), @p12='?', @p14='?'], CommandType='Text', CommandTimeout='0']INSERT INTO `rigofunc_2017`.`t_post_201703` (`Fcontent`, `Fis_deleted`, `Fuser_id`)VALUES (@p10, @p12, @p14);SELECT LAST_INSERT_ID();Executed DbCommand [Parameters=[@p0='?', @p3='?', @p4='?' (Size = 8000)], CommandType='Text', CommandTimeout='0']UPDATE `rigofunc_2017`.`t_user_201703` SET `Fpassword` = @p4WHERE `Fid` = @p0 AND `Fis_deleted` = @p3;SELECT ROW_COUNT();Executed DbCommand (1ms) [Parameters=[], CommandType='Text', CommandTimeout='0']SELECT `u`.`Fid`, `u`.`Fis_deleted`, `u`.`Fpassword`, `u`.`Fname`FROM `rigofunc_2017`.`t_user_201703` AS `u`ORDER BY `u`.`Fid`Executed DbCommand (2ms) [Parameters=[], CommandType='Text', CommandTimeout='0']SELECT `u`.`Fid`, `u`.`Fis_deleted`, `u`.`Fpassword`, `u`.`Fname`FROM `rigofunc_2018`.`t_user_201703` AS `u`ORDER BY `u`.`Fid`ABOUT ME：
I’m a software architect, particularly love .NET Core, but I also embrace all the new stuff. I’m on GitHub with xyting, and my packages publish on NuGet with rigofunc
原文地址：http://www.xyting.org/2017/03/25/EFCore-Support-MySQL-Multiple-Databases-And-Tables-sharding.html
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