如何在 EF Core 中实现 IModelCacheKeyFactory [英] How to implement IModelCacheKeyFactory in EF Core

问题描述

故事:在我们的多租户应用程序(一个 PostgreSql 数据库，多个架构)中，我们需要针对多个架构使用一个 DbContext.

The story: in our multi-tenant app (one PostgreSql db, multiple schemas) we need to use one DbContext against multiple schemas.

我尝试过的:持有缓存(字典，其中键是模式名称，值是该模式的上下文).当为另一个模式建立新的上下文时，我可以看到 dbContext 模式仍然设置为以前提供的模式.我假设上下文中的模型是按上下文类型在内部缓存的，所以这就是我看到这种行为的原因?

What I tried: holding a cache (Dictionary, where key is schema name, value is the context for that schema). When instatiating new context for another schema I can see that dbContext schema is still set to previous schema provided. I assume the model in context is cached internally by context type, so that is the reason I see this behavior?

以上似乎不起作用，我发现实现 IModelCacheKeyFactory 应该可以解决问题.有谁知道创建 方法呢?任何地方都没有示例或文档.

So above doesn't seem to work and I found that implementing IModelCacheKeyFactory should do the trick. Does anyone know what should go into Create method though? There are no samples nor documentation anywhere.

我发现了什么:在实体框架核心中动态改变架构，但它回答 EF6，所以帮助不大.

What I found: Dynamically changing schema in Entity Framework Core but it answers for EF6, so not much help.

推荐答案

这是一个例子.

派生的 DbContext，用自定义的替换它的 ModelCacheKey(和工厂).

Derived DbContext that replaces it's ModelCacheKey (and factory) with a Custom one.

class MyDbContext : DbContext
{
    public MyDbContext(string schema)
    {
        Schema = schema;
    }

    public string Schema { get; }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
        => options
            .UseSqlServer("...")
            .ReplaceService<IModelCacheKeyFactory, MyModelCacheKeyFactory>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.HasDefaultSchema(Schema);

        // ...
    }
}

使用特定键创建上下文的工厂.

The factory that creates the Context with a specific key.

class MyModelCacheKeyFactory : IModelCacheKeyFactory
{
    public object Create(DbContext context)
        => new MyModelCacheKey(context);
}

每个上下文的自定义 ModelCacheKey.

The custom ModelCacheKey per context.

class MyModelCacheKey : ModelCacheKey
{
    string _schema;

    public MyModelCacheKey(DbContext context)
        : base(context)
    {
        _schema = (context as MyDbContext)?.Schema;
    }

    protected override bool Equals(ModelCacheKey other)
        => base.Equals(other)
            && (other as MyModelCacheKey)?._schema == _schema;

    public override int GetHashCode()
    {
        var hashCode = base.GetHashCode() * 397;
        if (_schema != null)
        {
            hashCode ^= _schema.GetHashCode();
        }

        return hashCode;
    }
}

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