使用 Linq 语法调用数据库时，需要包含导航属性（外键表数据）会用到 Include 方法，但是如果引用的程序集搞错了，就不会有数据输出，应该：
using Microsoft.EntityFrameworkCore;
而不是
using System.Data.Entity;
| 引擎 | 连接方式 |
| mysqli | mysqli_connect("域名或IP:端口", "用户名", '密码', "数据库名") |
| Scaffold-DbContext | Scaffold-DbContext "server=域名或IP;port=端口;uid=用户名;pwd=密码;database=数据库名;" Pomelo.EntityFrameworkCore.MySql -OutputDir 模型路径 -Force |
如果端口配置不正确，会报错：
Unable to connect to any of the specified MySQL hosts.
Pomelo.EntityFrameworkCore.MySql 升级到 7.0 后出现：
System.InvalidOperationException:“The 'sbyte' property could not be mapped to the database type 'tinyint(1)' because the database provider does not support mapping 'sbyte' properties to 'tinyint(1)' columns. Consider mapping to a different database type or converting the property value to a type supported by the database using a value converter. See https://aka.ms/efcore-docs-value-converters for more information. Alternately, exclude the property from the model using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.”
对比 6.0 生成的 DbContext.cs 发现缺少了 UseLoggerFactory，按旧版修改即可。
找到 OnConfiguring 方法，上方插入：
public static readonly LoggerFactory MyLoggerFactory = new LoggerFactory(new[] {
new DebugLoggerProvider()
});在 OnConfiguring 方法中将：
optionsBuilder.UseMySql("连接字符串");改为：
optionsBuilder.UseLoggerFactory(MyLoggerFactory).UseMySql("连接字符串");开发环境正常，发布到 IIS 报错
Could not load file or assembly 'Microsoft.EntityFrameworkCore.Relational, Version=6.0.x.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'. 系统找不到指定的文件。
尝试在 UI 层安装 Pomelo.EntityFrameworkCore.MySql 无果。
最终，在发布时文件发布选项中，去掉“启用 ReadyToRun 编译”就正常了。
当然这种情况不是在所有启用了 ReadyToRun 编译的项目中遇到，但该项目改为不启用 ReadyToRun 后恢复正常了，原因暂时未知。
public static class EntityFrameworkCoreExtension
{
private static DbCommand CreateCommand(DatabaseFacade facade, string sql, out DbConnection connection, params object[] parameters)
{
var conn = facade.GetDbConnection();
connection = conn;
conn.Open();
var cmd = conn.CreateCommand();
cmd.CommandText = sql;
cmd.Parameters.AddRange(parameters);
return cmd;
}
public static DataTable SqlQuery(this DatabaseFacade facade, string sql, params object[] parameters)
{
var command = CreateCommand(facade, sql, out DbConnection conn, parameters);
var reader = command.ExecuteReader();
var dt = new DataTable();
dt.Load(reader);
reader.Close();
conn.Close();
return dt;
}
public static List<T> SqlQuery<T>(this DatabaseFacade facade, string sql, params object[] parameters) where T : class, new()
{
var dt = SqlQuery(facade, sql, parameters);
return dt.ToList<T>();
}
public static List<T> ToList<T>(this DataTable dt) where T : class, new()
{
var propertyInfos = typeof(T).GetProperties();
var list = new List<T>();
foreach (DataRow row in dt.Rows)
{
var t = new T();
foreach (PropertyInfo p in propertyInfos)
{
if (dt.Columns.IndexOf(p.Name) != -1 && row[p.Name] != DBNull.Value)
{
p.SetValue(t, row[p.Name], null);
}
}
list.Add(t);
}
return list;
}
}调用示例：
var data = db.Database.SqlQuery<List<string>>("SELECT 'title' FROM `table` WHERE `id` = {0};", id);using Microsoft.AspNetCore.Mvc;
using Microsoft.EntityFrameworkCore;
namespace xxx.xxx.xxx.Controllers
{
public class DiscuzTinyintViewerController : Controller
{
public IActionResult Index()
{
using var context = new Data.xxx.xxxContext();
var conn = context.Database.GetDbConnection();
conn.Open();
using var cmd = conn.CreateCommand();
cmd.CommandText = "SELECT `TABLE_NAME` FROM information_schema.TABLES WHERE TABLE_SCHEMA = DATABASE();";
Dictionary<string, List<FieldType>?> tables = new();
using var r = cmd.ExecuteReader();
while (r.Read())
{
tables.Add((string)r["TABLE_NAME"], null);
}
conn.Close();
foreach (var table in tables)
{
var conn2 = context.Database.GetDbConnection();
conn2.Open();
using var cmd2 = conn2.CreateCommand();
cmd2.CommandText = "DESCRIBE " + table.Key;
using var r2 = cmd2.ExecuteReader();
List<FieldType> fields = new();
while (r2.Read())
{
if (((string)r2[1]).Contains("tinyint(1)"))
{
fields.Add(new()
{
Field = (string)r2[0],
Type = (string)r2[1],
Null = (string)r2[2],
});
}
}
conn2.Close();
tables[table.Key] = fields;
}
foreach (var table in tables)
{
foreach (var f in table.Value)
{
var conn3 = context.Database.GetDbConnection();
conn3.Open();
using var cmd3 = conn3.CreateCommand();
cmd3.CommandText = $"SELECT {f.Field} as F, COUNT({f.Field}) as C FROM {table.Key} GROUP BY {f.Field}";
using var r3 = cmd3.ExecuteReader();
List<FieldType.ValueCount> vs = new();
while (r3.Read())
{
vs.Add(new() { Value = Convert.ToString(r3["F"]), Count = Convert.ToInt32(r3["C"]) });
}
conn3.Close();
f.groupedValuesCount = vs;
}
}
return Json(tables.Where(c => c.Value != null && c.Value.Count > 0));
}
private class FieldType
{
public string Field { get; set; }
public string Type { get; set; }
public string Null { get; set; }
public List<ValueCount> groupedValuesCount { get; set; }
public class ValueCount
{
public string Value { get; set; }
public int Count { get; set; }
}
public string RecommendedType
{
get
{
if (groupedValuesCount == null || groupedValuesCount.Count < 2)
{
return "无建议";
}
else if (groupedValuesCount.Count == 2 && groupedValuesCount.Any(c => c.Value == "0") && groupedValuesCount.Any(c => c.Value == "1"))
{
return "bool" + (Null == "YES" ? "?" : "");
}
else
{
return "sbyte" + (Null == "YES" ? "?" : "");
}
}
}
}
}
}[{
"key": "pre_forum_post",
"value": [{
"field": "first",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "0",
"count": 1395501
}, {
"value": "1",
"count": 179216
}],
"recommendedType": "bool"
}, {
"field": "invisible",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "-5",
"count": 9457
}, {
"value": "-3",
"count": 1412
}, {
"value": "-2",
"count": 1122
}, {
"value": "-1",
"count": 402415
}, {
"value": "0",
"count": 1160308
}, {
"value": "1",
"count": 3
}],
"recommendedType": "sbyte"
}, {
"field": "anonymous",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "0",
"count": 1574690
}, {
"value": "1",
"count": 27
}],
"recommendedType": "bool"
}, {
"field": "usesig",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "0",
"count": 162487
}, {
"value": "1",
"count": 1412230
}],
"recommendedType": "bool"
}, {
"field": "htmlon",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "0",
"count": 1574622
}, {
"value": "1",
"count": 95
}],
"recommendedType": "bool"
}, {
"field": "bbcodeoff",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "-1",
"count": 935448
}, {
"value": "0",
"count": 639229
}, {
"value": "1",
"count": 40
}],
"recommendedType": "sbyte"
}, {
"field": "smileyoff",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "-1",
"count": 1359482
}, {
"value": "0",
"count": 215186
}, {
"value": "1",
"count": 49
}],
"recommendedType": "sbyte"
}, {
"field": "parseurloff",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "0",
"count": 1572844
}, {
"value": "1",
"count": 1873
}],
"recommendedType": "bool"
}, {
"field": "attachment",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "0",
"count": 1535635
}, {
"value": "1",
"count": 2485
}, {
"value": "2",
"count": 36597
}],
"recommendedType": "sbyte"
}, {
"field": "comment",
"type": "tinyint(1)",
"null": "NO",
"groupedValuesCount": [{
"value": "0",
"count": 1569146
}, {
"value": "1",
"count": 5571
}],
"recommendedType": "bool"
}]
}]private static List<string> GetTableNames(DbContext context)
{
context.Database.OpenConnection();
var connection = context.Database.GetDbConnection();
using var command = connection.CreateCommand();
command.CommandText = @"SELECT `TABLE_NAME`
FROM information_schema.TABLES
WHERE TABLE_SCHEMA = DATABASE();";
var tableNames = new List<string>();
using var reader = command.ExecuteReader();
while (reader.Read())
{
tableNames.Add((string)reader["TABLE_NAME"]);
}
return tableNames;
}先上代码：
var db = new db_picContext();
var u = db.DtUser.Find(338);
int a = u.DtProject.Count();
int b = db.DtProject.Count(c => c.UploadUserId == u.Id);
return Json(new { a, b });运行结果：
{"a":0,"b":2}是不是说明 Find 结果 u 并不能通过导航属性（外键）来获取关联表的内容？
引擎：Pomelo.EntityFrameworkCore.MySql
本文使用 Oracle 官方提供的 MySql.EntityFrameworkCore，如使用 Pomelo.EntityFrameworkCore.MySql 请移步。
MySql.EntityFrameworkCore 是 MySql.Data.EntityFrameworkCore 的升级版
本文以 Visual Studio 2019、ASP.NET Core 5.0 开发环境为例。
新建 ASP.NET Core Web 应用程序。
安装 NuGet 包：
MySql.EntityFrameworkCore
Microsoft.EntityFrameworkCore.Design
根据已有数据库创建数据模型。在 NuGet 的程序包管理（Package Manager）控制台中（PowerShell）执行命令：
Scaffold-DbContext "server=数据库服务器;port=3306;user=数据库用户名;password=数据库密码;database=数据库名" MySql.EntityFrameworkCore -OutputDir Data -f.Net Core CLi：
dotnet ef dbcontext scaffold "server=数据库服务器;port=3306;user=数据库用户名;password=数据库密码;database=数据库名" MySql.EntityFrameworkCore -o Data -f搞定。
注：开发环境和生产环境都不需要安装 Connector/NET，只需要安装 ASP.NET Core。
补充：其它数据库提供程序请参考：https://docs.microsoft.com/zh-cn/ef/core/providers/
更多高级用法请参考官方文档。
您好，MySQL Connector/NET 社区，
从 8.0.23 版本开始，我们的 Entity Framework Core 提供程序有了一个新名称。主要目标是保持对 Microsoft Entity Framework Core 不同版本的支持，并确保这些版本与我们的发行版保持紧密联系。同样，此新命名在包装目的方面更为具体。因此，名称的“数据”部分已删除。
之前：
MySql.Data.EntityFrameworkCore v8.0.x
现在：
MySql.EntityFrameworkCore v8.0.x
现在，Microsoft 维护的不仅是 Entity Framework Core 的单个版本，我们还需要找到一种方法来命名程序包并维护 Entity Framework Core 和 MySQL 版本之间的相关性。这就是我们想到使用软件包元数据的时候。软件包的版本现在由两部分组成，第一部分对应于当前支持的 Microsoft Entity Framework Core 版本，然后是一个“加号”，指示我们包含 MySQL 版本的元数据。
MySql.EntityFrameworkCore v5.0.0+MySQL8.0.23 MySql.EntityFrameworkCore v3.1.10+MySQL8.0.23
好吧，还有另外一种未来情况需要考虑。如果 Microsoft 没有为我们的较新的 Connector/NET 版本及时发布 Entity Framework Core 的新版本怎么办？假设当前受支持的 Entity Framework Core 版本是5.0.0，所以我们的软件包看起来像这样 MySql.EntityFrameworkCore 5.0.0+MySQL8.0.23，并且在我们发布下一个版本 8.0.24 时，Microsoft Entity Framework Core 的最新版本仍然是 5.0.0。在这种情况下，我们将在版本中添加第四个数字，将包命名为：MySql.EntityFrameworkCore 5.0.0.1+MySQL8.0.23。
我们希望您发现此新的命名模型有用且更精确。始终欢迎您提供反馈，您的所有意见都会激励我们不断改进，以便我们为您提供高品质的产品。
最后，以下是一些可能对您有用的链接：
我们希望能与您保持联系！