.NET Core开发日志——Dapper与MySQL

Dapper做为.NET生态中广为人知的轻量级ORM类库在.NET Core里仍能被有效利用，而且其不但能够连通SQL Server数据库还提供对其它数据库，好比MySQL的支持。这里试验了一下经过Dapper链接MySQL的方法。

MySQL

能够选择直接安装在原生系统中或是Docker里。
Official
Docker

Table

在MySQL中创建两张表。

city表：

CREATE TABLE `city` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `Name` char(35) NOT NULL DEFAULT '',
  `CountryCode` char(3) NOT NULL DEFAULT '',
  `District` char(20) NOT NULL DEFAULT '',
  `Population` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`ID`),
  KEY `CountryCode` (`CountryCode`),
  CONSTRAINT `city_ibfk_1` FOREIGN KEY (`CountryCode`) REFERENCES `country` (`code`)
) ENGINE=InnoDB AUTO_INCREMENT=4080 DEFAULT CHARSET=latin1

country表：

CREATE TABLE `country` (
  `Code` char(3) NOT NULL DEFAULT '',
  `Name` char(52) NOT NULL DEFAULT '',
  `Continent` enum('Asia','Europe','North America','Africa','Oceania','Antarctica','South America') NOT NULL DEFAULT 'Asia',
  `Region` char(26) NOT NULL DEFAULT '',
  `SurfaceArea` float(10,2) NOT NULL DEFAULT '0.00',
  `IndepYear` smallint(6) DEFAULT NULL,
  `Population` int(11) NOT NULL DEFAULT '0',
  `LifeExpectancy` float(3,1) DEFAULT NULL,
  `GNP` float(10,2) DEFAULT NULL,
  `GNPOld` float(10,2) DEFAULT NULL,
  `LocalName` char(45) NOT NULL DEFAULT '',
  `GovernmentForm` char(45) NOT NULL DEFAULT '',
  `HeadOfState` char(60) DEFAULT NULL,
  `Capital` int(11) DEFAULT NULL,
  `Code2` char(2) NOT NULL DEFAULT '',
  PRIMARY KEY (`Code`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

Package

应用程序工程中须要添加Dapper以及Mysql.Data类库。

dotnet add package Dapper
dotnet add package MySql.Data

Entity

编写两个实体类，用于映射city与country表。

public class CityEntity
{
    public int ID { get; set; }
    public string Name { get; set; }
    public string CountryCode { get; set; }
    public string District { get; set; }
    public int Population { get; set; }
    public CountryEntity Country { get; set; }

    public override string ToString()
    {
        return $"ID: {ID}, Name: {Name}, CountryCode: {CountryCode}, District: {District}, Population: {Population}, Country: {Country}";
    }
}

public class CountryEntity
{
    public string Code { get; set; }
    public string Name { get; set; }
    public string Continent { get; set; }
    public string Region { get; set; }
    public decimal SurfaceArea { get; set; }
    public int IndepYear { get; set; }
    public int Population { get; set; }
    public decimal LifeExpectancy { get; set; }
    public decimal GNP { get; set; }
    public decimal GNPOld { get; set; }
    public string LocalName { get; set; }
    public string GovernmentForm { get; set; }
    public string HeadOfState { get; set; }
    public int Capital { get; set; }
    public string Code2 { get; set; }

    public override string ToString()
    {
        return $"Code: {Code}, Name: {Name}, Continent: {Continent}, Region: {Region}, SurfaceArea: {SurfaceArea} ";
    }
}

Repository

仓库类中新加获取10个城市数据的方法。这里Dapper的Query方法有三个参数，第一个是须要执行的SQL语句，第二个是对象之间的对应关系(这个例子中city与country为一对一关系)，并肯定最终返回的对象类型，最后的SplitOn参数会告诉Dapper在结果集中两张表之间以哪一个字段进行分界。

public class CityRepository
{
    public List<CityEntity> Get10Cities()
    {
        List<CityEntity> result;
        using (var conn = new MySqlConnection("Host=localhost;Port=3306;Database=world;Uid=admin;pwd=admin"))
        {
            var sql = "SELECT * FROM city INNER JOIN country ON city.CountryCode = country.Code LIMIT 10";
            result = conn.Query<CityEntity, CountryEntity, CityEntity>(sql,
                (city, country) => { city.Country = country; return city; }, splitOn: "Code").ToList();
        }

        return result;
    }
}

Test

static void Main(string[] args)
{
    var repository = new CityRepository();
    var cities = repository.Get10Cities();
    cities.ForEach(e=>{
        System.Console.WriteLine(e);
    });
}

程序运行的结果以下，能够看到成功借助Dapper的力量从MySQL数据库里获取了所需的数据。