EF Core3.1 CodeFirst动态自动添加表和字段的描述信息

前言

我又来啦..

本篇主要记录如何针对CodeFirst做自动添加描述的扩展

为什么要用这个呢.. 因为EF Core3.1 CodeFirst 对于自动添加描述这块 只有少部分的数据库支持..

然而我们的客户大佬们 对这个又有要求..所以..没办法 只能自己扩展~

当然也可以根据这个原理来做一些有意思的扩展~

本文就以不支持的达梦数据库来举个栗子

.(PS:真心希望达梦数据库能开放EF Core相关的源码,这样我们也好提交点贡献,国产数据库还是不能太过敝帚自珍阿..)

正文

1.通过扩展生成器,来实现动态自动添加描述信息

我们知道在SQL Server中,可以通过Fluent API来添加针对表或者字段的描述,如下:

builder.Property(prop.Name)
    .HasComment("XXX字段描述");

然而在达梦的上下文中,我们如果这样写..是没任何效果的..不用想,肯定是达梦的开发商没写(很多扩展类都缺斤少两)..

那就需要我们自己扩展了, 所以就少不了翻看EF Core源码..

我们通过翻看源码,可以找到MigrationsSqlGenerator这个类. 类名翻译过来,喔唷,这不就是迁移SQL生成器么

那么我们就需要去实现他啦.首先,我们找到达梦实现他的子类:DmMigrationsSqlGenerator

通过反编译,我们发现,果然他并没实现对于Comment属性的代码,那么我们就需要自行扩展

我们添加MyDmigrationsSqlGenerator类继承DmMigrationsSqlGenerator 添加扩展代码如下:

using Microsoft.EntityFrameworkCore.Metadata;
using Microsoft.EntityFrameworkCore.Migrations;
using Microsoft.EntityFrameworkCore.Migrations.Operations;
using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Linq;
using System.Text;

namespace Ciac.ZTBExpert.Model
{
    public class MyDmigrationsSqlGenerator : DmMigrationsSqlGenerator
    {
        public MyDmigrationsSqlGenerator([NotNull] MigrationsSqlGeneratorDependencies dependencies, [NotNull] IMigrationsAnnotationProvider migrationsAnnotations)
        : base(dependencies, migrationsAnnotations)
        {

        }

        protected override void Generate(
           CreateTableOperation operation,
           IModel model,
           MigrationCommandListBuilder builder,
           bool terminate)
        {
            base.Generate(operation, model, builder, terminate);
            var comment = operation.Comment;
            if (comment != null)
            {
                if (terminate)
                {
                    builder.AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator);
                    EndStatement(builder);
                }
                builder
                    .Append("COMMENT ON TABLE ")
                    .Append(Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Name, operation.Schema))
                    .Append(" IS ")
                    .Append($"'{comment}'")
                    .Append(Dependencies.SqlGenerationHelper.StatementTerminator);
            }
            // Comments on the columns
            foreach (var columnOp in operation.Columns.Where(c => c.Comment != null))
            {
                var columnComment = columnOp.Comment;
               // builder.AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator);
                EndStatement(builder);
                builder
                    .Append("COMMENT ON COLUMN ")
                    .Append(Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Name, operation.Schema))
                    .Append('.')
                    .Append(Dependencies.SqlGenerationHelper.DelimitIdentifier(columnOp.Name))
                    .Append(" IS ")
                    .Append($"'{columnComment}'")
                    .Append(Dependencies.SqlGenerationHelper.StatementTerminator);
            }
            builder.EndCommand();
        }


        protected override void Generate(AlterColumnOperation operation, IModel model, MigrationCommandListBuilder builder)
        {
            base.Generate(operation, model, builder);
            // Comment
            var oldComment = operation.OldColumn.Comment;
            var newComment = operation.Comment;

            if (oldComment != newComment)
            {
                //builder.AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator);

                builder
                    .Append("COMMENT ON COLUMN ")
                    .Append(Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Table, operation.Schema))
                    .Append('.')
                    .Append(Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Name))
                    .Append(" IS ")
                    .Append($"'{newComment}'")
                    .Append(Dependencies.SqlGenerationHelper.StatementTerminator);
                builder.EndCommand();
            }

        }

        protected override void Generate(AddColumnOperation operation, IModel model, MigrationCommandListBuilder builder, bool terminate)
        {
            base.Generate(operation, model, builder, terminate);
            // Comment
            var newComment = operation.Comment;

            if (!string.IsNullOrEmpty(newComment))
            {
               // builder.AppendLine(Dependencies.SqlGenerationHelper.StatementTerminator);
                builder
                    .Append("COMMENT ON COLUMN ")
                    .Append(Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Table, operation.Schema))
                    .Append('.')
                    .Append(Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Name))
                    .Append(" IS ")
                    .Append($"'{newComment}'")
                    .Append(Dependencies.SqlGenerationHelper.StatementTerminator);
                if (terminate)
                {
                    EndStatement(builder);
                }
                builder.EndCommand();

            }

        }
        protected override void Generate([NotNull] AlterTableOperation operation, IModel? model, [NotNull] MigrationCommandListBuilder builder)
        {
            base.Generate(operation, model, builder);
            // Comment
            var oldComment = operation.OldTable.Comment;
            var newComment = operation.Comment;

            if (oldComment != newComment)
            {
                
                EndStatement(builder);

                builder
                    .Append("COMMENT ON TABLE ")
                    .Append(Dependencies.SqlGenerationHelper.DelimitIdentifier(operation.Name, operation.Schema))
                    .Append(" IS ")
                    .Append($"'{newComment}'")
                    .Append(Dependencies.SqlGenerationHelper.StatementTerminator);
                builder.EndCommand();
            }
        }
    }
}

因为我们只是想在创建或者修改表后添加描述.

所以,我们只需要针对CreateTable,AlterColumn,AddColumn,AlterTable 四个生成方法做重写就好了

最后我们需要在EF上下文初始化之前来替换掉原来的生成器如下:

这样,我们就可以通过在上下文中配置Fluent API就可以自动生成描述了~

我们在EF上下文的OnModelCreating添加代码如下:

 protected override void OnModelCreating(ModelBuilder modelBuilder)
 {
            modelBuilder.Entity<tab_zjcq_ggxx>(a => a.Property("aaa").HasComment("88888"));
}

执行迁移语句Script-Migration

结果如下:

ALTER TABLE "tab_zjcq_ggxx" MODIFY "aaa" NVARCHAR2(50) NULL;


/COMMENT ON COLUMN "tab_zjcq_ggxx"."aaa" IS '8888';

2.通过添加Description特性来优化代码风格,方便管理

虽然上面第一步就已经实现了我们的要求,但是我们发现,通过Fluent API 来添加描述,代码可读性会很差,

且一旦表多起来,那么OnModelCreating 方法就会变的超长(虽然也可以写在实体类里面,但是就觉得很麻烦)..

那么能不能像[MaxLength(50)] 这种特性一样,直接在字段上加个特性来解决这个事情呢?~

当然是可以的啦~

我们修改OnModelCreating 中的代码如下:

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            
            var ddd= modelBuilder.Model.GetEntityTypes().ToList();
            foreach (var item in ddd)
            {
                var tabtype = Type.GetType(item.ClrType.FullName);
                var props = tabtype.GetProperties();
                var descriptionAttrtable = tabtype.GetCustomAttributes(typeof(DescriptionAttribute), true);
                if (descriptionAttrtable.Length > 0)
                {
                    modelBuilder.Entity(item.Name).HasComment(((DescriptionAttribute)descriptionAttrtable[0]).Description);
                }
                foreach (var prop in props)
                {
                    var descriptionAttr = prop.GetCustomAttributes(typeof(DescriptionAttribute), true);
                    if (descriptionAttr.Length>0)
                    {
                        modelBuilder.Entity(item.Name).Property(prop.Name).HasComment(((DescriptionAttribute)descriptionAttr[0]).Description);
                    }
                }
            }

        }

这里通过反射,得到包含DescriptionAttribute特性的字段,然后读取描述信息,通过HasComment 自动添加~

然后我们给字段添加描述如下:

执行迁移语句Script-Migration~

我们会发现,描述已经自动生成啦~

结束语

其实不管是.NET 5.0 还是EF Core 在开源化的今天,我们只要愿意去多翻翻源码,会发现自己可以扩展的东西还有很多~很多~

最后..在提一嘴,真心希望国产数据库的访问库 能够开源.. 毕竟,人多力量大~

又不需要数据库应用开源..起码访问组件 你能开源吧..

好了..就到这了 瑞思拜~