八、Entity Framework Core 3.1入门教程-查询关系数据

时间 2020-09-14

标签 entity framework core 3.1 入门教程查询关系数据繁體版

原文原文链接

本文章是根据微软MVP solenovex（杨旭）老师的视频教程编写而来，再加上本身的一些理解。
视频教程地址：https://www.bilibili.com/video/BV1xa4y1v7rR
GitHub源码：https://github.com/hllive/LearnEFCore3.1git

预加载（Eager Loading）

把全部俱乐部和俱乐部的联赛所有查询出来，使用Include()方法关联起来github

[HttpGet("Eager")]
public IActionResult GetEager()
{
    var clubs = _dbContext.Clubs
        .Include(c => c.League)//关联数据
        .ToList();
    return Ok(clubs);
}

生成的SQL语句

执行结果
sql

这样查询全部数据是没有意义的，咱们能够添加过滤条件，过滤条件在DbSet()后添加Where()方法；json

[HttpGet("Eager")]
public IActionResult GetEager()
{
    var clubs = _dbContext.Clubs
        .Where(c => c.Name.Contains("足球队"))//过滤条件
        .Include(c => c.League)//关联数据
        .ToList();
    return Ok(clubs);
}

注意！不能将ToList()或FirstDefault()放在Include()的前面，Include()只针对IQueryable返回类型才有Include()方法；另外DbSet()的Find()方法也不支持Include()ide

如今能够把俱乐部的全部队员加载进来，每一个队员也有简历关联，关联简历使用ThenInclude()方法（级联添加关系数据）3d

Include是针对Clubs的关联，子属性须要关联就使用ThenInclude()方法
若是队员还须要关联GamePlayer的话，再使用ThenInclude()方法是不行的，在继续使用ThenInclude()方法的话是针对Resume的关联。code

[HttpGet("Eager")]
public IActionResult GetEager()
{
    var clubs = _dbContext.Clubs
        .Where(c => c.Name.Contains("足球队"))   //过滤条件
        .Include(c => c.League)                 //关联数据-联赛
        .Include(c => c.Players)                //关联数据-队员
            .ThenInclude(p => p.Resume)         //关联子属性的简历
        .Include(c => c.Players)                //继续关联数据-队员
            .ThenInclude(p => p.GamePlayers)    //关联子属性
                .ThenInclude(g => g.Game)       //GamePlayers关联Game子属性
        .ToList();
    return Ok(clubs);
}

最终生成的SQL语句视频

SELECT [c].[Id], [c].[City], [c].[DateOfEstablishment], [c].[History], [c].[LeagueId], [c].[Name], [l].[Id], [l].[Country], [l].[Name], [t0].[Id], [t0].[Birth], [t0].[ClubId], [t0].[Name], [t0].[ResumeId], [t0].[Id0], [t0].[Description], [t0].[PlayerId], [t0].[PlayerId0], [t0].[GameId], [t0].[Id1], [t0].[Round], [t0].[StartTime]
FROM [Clubs] AS [c]
LEFT JOIN [Leagues] AS [l] ON [c].[LeagueId] = [l].[Id]
LEFT JOIN (
    SELECT [p].[Id], [p].[Birth], [p].[ClubId], [p].[Name], [p].[ResumeId], [r].[Id] AS [Id0], [r].[Description], [r].[PlayerId], [t].[PlayerId] AS [PlayerId0], [t].[GameId], [t].[Id] AS [Id1], [t].[Round], [t].[StartTime]
    FROM [Players] AS [p]
    LEFT JOIN [Resumes] AS [r] ON [p].[Id] = [r].[PlayerId]
    LEFT JOIN (
        SELECT [g].[PlayerId], [g].[GameId], [g0].[Id], [g0].[Round], [g0].[StartTime]
        FROM [GamePlayers] AS [g]
        INNER JOIN [Games] AS [g0] ON [g].[GameId] = [g0].[Id]
    ) AS [t] ON [p].[Id] = [t].[PlayerId]
) AS [t0] ON [c].[Id] = [t0].[ClubId]
WHERE CHARINDEX(N'足球队', [c].[Name]) > 0
ORDER BY [c].[Id], [t0].[Id], [t0].[PlayerId0], [t0].[GameId], [t0].[Id1]

最终查询出来的数据对象

[
    {
        "id": "21ec89ad-1b66-4b65-03e4-08d845203d1f",
        "name": "茅台足球队",
        "city": "贵州仁怀",
        "dateOfEstablishment": "1999-07-01T00:00:00",
        "history": null,
        "league": {
            "id": "edaaee79-78c9-43b5-a924-08d845203d11",
            "name": "遵义仁怀足球联赛",
            "country": null
        },
        "players": []
    },
    {
        "id": "5d711109-2807-472d-6c10-08d847725f25",
        "name": "新智联足球队",
        "city": "贵州省贵阳市",
        "dateOfEstablishment": "2020-08-23T00:00:00",
        "history": "参加不少比赛",
        "league": {
            "id": "4227506d-05e4-47a2-b94f-08d8451d5dc0",
            "name": "第一季度足球联赛-",
            "country": "中国"
        },
        "players": [
            {
                "id": "fa896d64-e87c-4087-4e18-08d847725f2b",
                "name": "王建国",
                "birth": "1994-08-02T00:00:00",
                "gamePlayers": [],
                "resumeId": "00000000-0000-0000-0000-000000000000",
                "resume": null
            },
            {
                "id": "916ea175-5aa9-4249-4e19-08d847725f2b",
                "name": "李刚",
                "birth": "1994-09-25T00:00:00",
                "gamePlayers": [],
                "resumeId": "00000000-0000-0000-0000-000000000000",
                "resume": null
            },
            {
                "id": "3fed8a06-6e9a-461e-2249-08d847cd3d8b",
                "name": "陈浩杰",
                "birth": "2000-05-06T00:00:00",
                "gamePlayers": [],
                "resumeId": "00000000-0000-0000-0000-000000000000",
                "resume": null
            }
        ]
    }
]

继续看例子，经过Linq表达式，至关于SQL语句的写法，选择不一样的字段；使用Select()方法选择一些字段，使用匿名类，匿名类中包括本身的一些属性和关联属性的属性blog

[HttpGet("Eager2")]
public IActionResult GetEager2()
{
    var clubs = _dbContext.Clubs
        .Where(c => c.Name.Contains("足球队"))   //过滤条件
        .Select(x => new
        {
            x.Id,//本身的属性
            x.Name,//本身的属性
            LeagueName = x.League.Name,//关联属性的属性
            Players = x.Players.Where(p => p.Birth > new DateTime(2000, 1, 1))//查询过滤条件的队员
        })
        .ToList();
    return Ok(clubs);
}

生成的SQL语句

SELECT [c].[Id], [c].[Name], [l].[Name], [t].[Id], [t].[Birth], [t].[ClubId], [t].[Name], [t].[ResumeId]
FROM [Clubs] AS [c]
LEFT JOIN [Leagues] AS [l] ON [c].[LeagueId] = [l].[Id]
LEFT JOIN (
    SELECT [p].[Id], [p].[Birth], [p].[ClubId], [p].[Name], [p].[ResumeId]
    FROM [Players] AS [p]
    WHERE [p].[Birth] > '2000-01-01'
) AS [t] ON [c].[Id] = [t].[ClubId]
WHERE CHARINDEX(N'足球队', [c].[Name]) > 0
ORDER BY [c].[Id], [t].[Id]

查询出来的数据

[
    {
        "id": "21ec89ad-1b66-4b65-03e4-08d845203d1f",
        "name": "茅台足球队",
        "leagueName": "遵义仁怀足球联赛",
        "players": []
    },
    {
        "id": "5d711109-2807-472d-6c10-08d847725f25",
        "name": "新智联足球队",
        "leagueName": "第一季度足球联赛-",
        "players": [
            {
                "id": "3fed8a06-6e9a-461e-2249-08d847cd3d8b",
                "name": "陈浩杰",
                "birth": "2000-05-06T00:00:00",
                "gamePlayers": [],
                "resumeId": "00000000-0000-0000-0000-000000000000",
                "resume": null
            }
        ]
    }
]

这种查询出来的结果是一个匿名类，匿名类Context不能进行变化追踪，只能追踪它识别的类=>DbSet ()
可是匿名类中包括Context识别的类，上例子中的Players类是能够被Context识别并追踪的，这种也能够进行变化追踪操做。

显式加载（Explicit Loading）

经过对象逐一查询关联数据

[HttpGet("Explicit")]
public IActionResult GetExplicit()
{
    //一、查询一条俱乐部数据
    var club = _dbContext.Clubs.FirstOrDefault();
    //二、经过查询出来的对象逐一查询关联数据-队员
    _dbContext.Entry(club)
        .Collection(x => x.Players)//关联队员集合数据
        .Load();
    //三、经过查询出来的对象逐一查询关联数据-联赛
    _dbContext.Entry(club)
        .Reference(x => x.League)//关联单个联赛数据
        .Load();
    return Ok(club);
}

从结果中看出执行了3次SQL语句查询

这种方法只能针对单个数据进行逐一查询关联数据，也就是单个俱乐部，若是针对list集合这种方法就不行。

也能够给查询语句添加过滤条件

[HttpGet("Explicit")]
public IActionResult GetExplicit()
{
    //一、查询一条俱乐部数据
    var club = _dbContext.Clubs.FirstOrDefault();
    //二、经过查询出来的对象逐一查询关联数据-队员
    _dbContext.Entry(club)
        .Collection(x => x.Players)//关联队员集合数据
        .Query().Where(x => x.Birth > new DateTime(2000, 1, 1))//添加过滤条件
        .Load();
    //三、经过查询出来的对象逐一查询关联数据-联赛
    _dbContext.Entry(club)
        .Reference(x => x.League)//关联单个联赛数据
        .Load();
    return Ok(club);
}

懒加载（Lazy Loading）

这种特性在EFCore中默认是关闭的，懒加载会遇到不少问题

其余查询

一、使用关联对象的一些属性做为查询过滤条件，如下是经过俱乐部关联的联赛按联赛的过滤查询条件获取数据

var data = _dbContext.Clubs.Where(x => x.League.Name.Contains("足球联赛"));

二、查询多对多关系
因为GamePlayer没在Context的DbSet属性里，但实际是被追踪的，可使用context.set<GamePlayer>()，若是遇到在Context的DbSet属性没有体现的类，可使用context.set<class>()追踪

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