实体框架4：如何查找主键？

I am trying to create a generic method using EF4 to find the primary key of an object.

我正在尝试使用EF4创建一个通用方法来查找对象的主键。

example

public string GetPrimaryKey<T>()
{
    ...
}

To give more info I am working off of the Tekpub StarterKit and below is the class I am trying to get up and running

为了提供更多信息,我正在使用Tekpub StarterKit,下面是我试图启动和运行的课程

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data.Objects;
using System.Data.Objects.ELinq;
using System.Data.Linq;
using Web.Infrastructure.Storage.EF4;

namespace Web.Infrastructure.Storage {
public class EFSession:ISession {
    PuzzleEntities _db;//This is an ObjectContext
    public EFSession() {
        _db = new PuzzleEntities();
    }

    public void CommitChanges() {
        _db.SaveChanges();
    }
    /// <summary>
    /// Gets the table provided by the type T and returns for querying
    /// </summary>
    private ObjectSet<T> GetObjectSet<T>() where T:class {
        return _db.CreateObjectSet<T>();
    }

    private T GetByPrimaryKey<T>() where T: class
    {
        .....
    }

    public void Delete<T>(System.Linq.Expressions.Expression<Func<T, bool>> expression) where T: class{

        foreach (T item in All<T>().Where(expression))
        {
            GetObjectSet<T>().DeleteObject(item);
        }
    }

    public void Delete<T>(T item) where T : class {
        GetObjectSet<T>().DeleteObject(item);
    }

    public void DeleteAll<T>() where T : class {
        foreach(T item in All<T>())
        {
            GetObjectSet<T>().DeleteObject(item);
        }
    }

    public void Dispose() {
        _db.Dispose();
    }

    public T Single<T>(System.Linq.Expressions.Expression<Func<T, bool>> expression) where T:class {
        return GetObjectSet<T>().SingleOrDefault(expression);
    }

    public IQueryable<T> All<T>() where T : class {
        return GetObjectSet<T>().AsQueryable();
    }

    public void Add<T>(T item) where T : class {
        GetObjectSet<T>().AddObject(item);
    }
    public void Add<T>(IEnumerable<T> items) where T : class {
        foreach (T item in items)
        {
            GetObjectSet<T>().AddObject(item);
        }
    }
    public void Update<T>(T item) where T : class {
        //nothing needed here
    }
}
}

4 个解决方案

#1

So I was finally able to find out how to get this to work. I wish I hadn't lost the link to the blog I read last night as I didn't write the code.

所以我终于能够找到如何使这个工作。我希望我没有丢失我昨晚读到的博客的链接,因为我没有写代码。

public T GetByPrimaryKey<T>(int id) where T : class
{
    return (T)_db.GetObjectByKey(new EntityKey(_db.DefaultContainerName + "." + this.GetEntityName<T>(), GetPrimaryKeyInfo<T>().Name, id));
}

string GetEntityName<T>()
{
    string name = typeof(T).Name;
    if (name.ToLower() == "person")
        return "People";
    else if (name.Substring(name.Length - 1, 1).ToLower() == "y")
        return name.Remove(name.Length - 1, 1) + "ies";
    else if (name.Substring(name.Length - 1, 1).ToLower() == "s")
        return name + "es";
    else
        return name + "s";
}

private PropertyInfo GetPrimaryKeyInfo<T>()
{
    PropertyInfo[] properties = typeof(T).GetProperties();
    foreach (PropertyInfo pI in properties)
    {
        System.Object[] attributes = pI.GetCustomAttributes(true);
        foreach (object attribute in attributes)
        {
            if (attribute is EdmScalarPropertyAttribute)
            {
                if ((attribute as EdmScalarPropertyAttribute).EntityKeyProperty == true)
                    return pI;
            }
            else if (attribute is ColumnAttribute)
            {

                if ((attribute as ColumnAttribute).IsPrimaryKey == true)
                    return pI;
            }
        }
    }
    return null;
}

I hope this helps someone else. All I can say is that it should be a little clearer on how to do this.

我希望这有助于其他人。我只能说,如何做到这一点应该更清楚一些。

#2

There is a property on each EF4 entity called EntityKey which contains an array of EntityKeyValues (array is there in case of compound key).

每个名为EntityKey的EF4实体都有一个属性,它包含一个EntityKeyValues数组(如果是复合键,则为数组)。

You could reference this directly on your entity instance or create a generic helper method that does this under the covers. If I can test-drive some sample code, I'll post it up here.

您可以直接在您的实体实例上引用它,或者创建一个通用的帮助方法来实现这一点。如果我可以测试一些示例代码,我会在这里发布。

Edit: The EntityKeyValue is a KeyValuePair<TKey, TValue> where the key is the primary key field of the entity and the value is the associated value.

编辑:EntityKeyValue是KeyValuePair ,其中键是实体的主键字段,值是关联值。 ,tvalue>

E.g., I have an entity called Company whose primary key is the field Symbol.

例如,我有一个名为Company的实体,其主键是字段Symbol。

var firstCompany = (from c in context.Companies select c).FirstOrDefault();
var kvp = firstCompany.EntityKey.EntityKeyValues[0];
// kvp shows {[Symbol, FOO]}

In my sandbox, I noticed this property was null when I created the entity in code. But once I read the entity from the database, it was correctly populated. So, it appears that the EF4 concept of a primary key only comes in to play once it hits the database. Although, you are free to set it explicitly ahead of time, if you wish.

在我的沙箱中,我注意到当我在代码中创建实体时,此属性为null。但是一旦我从数据库中读取实体,它就被正确填充了。因此,似乎主键的EF4概念只有在它到达数据库时才能发挥作用。虽然如果您愿意,可以提前明确地设置它。

#3

I assume many people stop by this post just by looking "Entity framework how to find the primary key?" regardless EF version (like me). So I wanted to mentioned that with EF 6.1, you can also create an extension methods to get the primary key. Following is the example and works perfectly fine.

我假设很多人只是通过查看“实体框架如何找到主键?”来阻止这篇文章。无论EF版本(像我一样)。所以我想提一下,使用EF 6.1,您还可以创建扩展方法来获取主键。以下是示例,完美无缺。

PS: I am not 100% sure, if that would work with composite and compound keys tho.

PS:我不是100%肯定,如果那可以使用复合键和复合键。

using System;
using System.Data.Entity;
using System.Data.Entity.Core.Metadata.Edm;
using System.Data.Entity.Infrastructure;
using System.Linq;

namespace System.Data.Entity
{
    public static class DbContextExtensions
    {
        public static string[] GetKeyNames<TEntity>(this DbContext context)
            where TEntity : class
        {
            return context.GetKeyNames(typeof(TEntity));
        }

        public static string[] GetKeyNames(this DbContext context, Type entityType)
        {
            var metadata = ((IObjectContextAdapter)context).ObjectContext.MetadataWorkspace;

            // Get the mapping between CLR types and metadata OSpace
            var objectItemCollection = ((ObjectItemCollection)metadata.GetItemCollection(DataSpace.OSpace));

            // Get metadata for given CLR type
            var entityMetadata = metadata
                    .GetItems<EntityType>(DataSpace.OSpace)
                    .Single(e => objectItemCollection.GetClrType(e) == entityType);

            return entityMetadata.KeyProperties.Select(p => p.Name).ToArray();
        }
    }
}

Original Source

#4

this seems needlessly long? I have had the same need, and using the suggestions above (by SethO and denis_n), i am using:

这似乎是不必要的长?我有同样的需求,并使用上面的建议(由SethO和denis_n),我正在使用:

        //get the primary key field name and location for the table
        var primaryFieldName = entry.EntitySet.ElementType.KeyMembers[0].Name ;
        int primaryFieldLocation = entry.CurrentValues.GetOrdinal(primaryFieldName);
        //gets the value pair for the primary key (returns field name + value)
        var primaryField = entry.EntityKey.EntityKeyValues[primaryFieldLocation];
        String primaryFieldValue = primaryField.Value.ToString();

Hope this helps anyone who is interested

希望这可以帮助任何有兴趣的人