EF4 LINQ包含(字符串)硬编码字符串的替代方法？

Question

有没有替代方案:

Organizations.Include("Assets").Where(o => o.Id == id).Single()

我希望看到类似的东西:

Organizations.Include(o => o.Assets).Where(o => o.Id == id).Single()

避免使用硬编码字符串"Assets".

Answer 1

在EF 4.1中,有一个内置的扩展方法.

您必须在项目中引用"EntityFramework"程序集(EF 4.1所在的位置)并使用System.Data.Entity.

using System.Data.Entity; 

如果要包含嵌套实体,可以这样做:

        db.Customers.Include(c => c.Orders.Select(o => o.LineItems))

不确定这是否适用于EF4.0实体(基于ObjectContext).

Answer 2

对于Entity Framework 1.0,我为此创建了一些扩展方法.

public static class EntityFrameworkIncludeExtension
{
    public static ObjectQuery<T> Include<T>(this ObjectQuery<T> src, Expression<Func<T, StructuralObject>> fetch)
    {
        return src.Include(CreateFetchingStrategyDescription(fetch));
    }

    public static ObjectQuery<T> Include<T>(this ObjectQuery<T> src, Expression<Func<T, RelatedEnd>> fetch)
    {
        return src.Include(CreateFetchingStrategyDescription(fetch));
    }

    public static ObjectQuery<T> Include<T, TFectchedCollection>(this ObjectQuery<T> src, Expression<Func<T, IEnumerable<TFectchedCollection>>> fetch)
    {
        return src.Include(CreateFetchingStrategyDescription(fetch));
    }

    public static ObjectQuery<T> Include<T, FetchedChild>(this ObjectQuery<T> src, Expression<Func<T, RelatedEnd>> fetch, Expression<Func<FetchedChild, Object>> secondFetch)
        where FetchedChild : StructuralObject
    {
        return src.Include(CombineFetchingStrategies(fetch, secondFetch));
    }

    public static ObjectQuery<T> Include<T, FetchedChild>(this ObjectQuery<T> src, Expression<Func<T, RelatedEnd>> fetch, Expression<Func<FetchedChild, RelatedEnd>> secondFetch)
        where FetchedChild : StructuralObject
    {
        return src.Include(CombineFetchingStrategies(fetch, secondFetch));
    }

    public static ObjectQuery<T> Include<T, FetchedChild>(this ObjectQuery<T> src, Expression<Func<T, RelatedEnd>> fetch, Expression<Func<FetchedChild, StructuralObject>> secondFetch)
        where FetchedChild : StructuralObject
    {
        return src.Include(CombineFetchingStrategies(fetch, secondFetch));
    }

    public static ObjectQuery<T> Include<T, FetchedChild>(this ObjectQuery<T> src, Expression<Func<T, IEnumerable<FetchedChild>>> fetch, Expression<Func<FetchedChild, Object>> secondFetch)
        where FetchedChild : StructuralObject
    {
        return src.Include(CombineFetchingStrategies(fetch, secondFetch));
    }

    public static ObjectQuery<T> Include<T, FetchedChild>(this ObjectQuery<T> src, Expression<Func<T, IEnumerable<FetchedChild>>> fetch, Expression<Func<FetchedChild, RelatedEnd>> secondFetch)
        where FetchedChild : StructuralObject
    {
        return src.Include(CombineFetchingStrategies(fetch, secondFetch));
    }

    public static ObjectQuery<T> Include<T, FetchedChild>(this ObjectQuery<T> src, Expression<Func<T, IEnumerable<FetchedChild>>> fetch, Expression<Func<FetchedChild, StructuralObject>> secondFetch)
        where FetchedChild : StructuralObject
    {
        return src.Include(CombineFetchingStrategies(fetch, secondFetch));
    }

    private static String CreateFetchingStrategyDescription<TFetchEntity, TFetchResult>(
        Expression<Func<TFetchEntity, TFetchResult>> fetch)
    {
        fetch = (Expression<Func<TFetchEntity, TFetchResult>>)FixedWrappedMemberAcces.ForExpression(fetch);
        if (fetch.Parameters.Count > 1)
            throw new ArgumentException("CreateFetchingStrategyDescription support only " +
                "one parameter in a dynamic expression!");

        int dot = fetch.Body.ToString().IndexOf(".") + 1;
        return fetch.Body.ToString().Remove(0, dot);
    }

    private static String CreateFetchingStrategyDescription<T>(Expression<Func<T, Object>> fetch)
    {
        return CreateFetchingStrategyDescription<T, Object>(fetch);
    }

    private static String CombineFetchingStrategies<T, TFetchedEntity>(
                Expression<Func<T, Object>> fetch, Expression<Func<TFetchedEntity, Object>> secondFetch)
    {
        return CombineFetchingStrategies<T, Object, TFetchedEntity, Object>(fetch, secondFetch);
    }

    private static String CombineFetchingStrategies<TFetchEntity, TFetchResult, TFetchedEntity, TSecondFetchResult>(
        Expression<Func<TFetchEntity, TFetchResult>> fetch, Expression<Func<TFetchedEntity, TSecondFetchResult>> secondFetch)
    {
        return CreateFetchingStrategyDescription<TFetchEntity, TFetchResult>(fetch) + "." +
            CreateFetchingStrategyDescription<TFetchedEntity, TSecondFetchResult>(secondFetch);
    }
}

用法:

 Orders.Include(o => o.Product); // generates .Include("Product")
 Orders.Include(o => o.Product.Category); // generates .Include("Product.Category")
 Orders.Include(o => o.History); // a 1-* reference => .Include("History")
 // fetch all the orders, and in the orders collection.
 // also include the user reference so: .Include("History.User")
 // but because history is an collection you cant write o => o.History.User, 
 // there is an overload which accepts a second parameter to describe the fetching 
 // inside the collection.
 Orders.Include(o => o.History, h => h.User); 

我没有在EF4.0上测试过这个,但我希望它可以工作.

Answer 3

使用表达式,这很容易做到:

public static class ObjectQueryExtensions
{
    public static ObjectQuery<T> Include<T, TProperty>(this ObjectQuery<T> objectQuery, Expression<Func<T, TProperty>> selector)
    {
        MemberExpression memberExpr = selector.Body as MemberExpression;
        if (memberExpr != null)
        {
            return objectQuery.Include(memberExpr.Member.Name);
        }
        throw new ArgumentException("The expression must be a MemberExpression", "selector");
    }
}

您可以完全按照问题中的示例使用它

UPDATE

改进版本,支持多个链接属性:

public static class ObjectQueryExtensions
{
    public static ObjectQuery<T> Include<T, TProperty>(this ObjectQuery<T> objectQuery, Expression<Func<T, TProperty>> selector)
    {
        string propertyPath = GetPropertyPath(selector);
        return objectQuery.Include(propertyPath);
    }

    public static string GetPropertyPath<T, TProperty>(Expression<Func<T, TProperty>> selector)
    {
        StringBuilder sb = new StringBuilder();
        MemberExpression memberExpr = selector.Body as MemberExpression;
        while (memberExpr != null)
        {
            string name = memberExpr.Member.Name;
            if (sb.Length > 0)
                name = name + ".";
            sb.Insert(0, name);
            if (memberExpr.Expression is ParameterExpression)
                return sb.ToString();
            memberExpr = memberExpr.Expression as MemberExpression;
        }
        throw new ArgumentException("The expression must be a MemberExpression", "selector");
    }
}

示例:

var query = X.Include(x => x.Foo.Bar.Baz) // equivalent to X.Include("Foo.Bar.Baz")