在linq中对实体使用自定义方法 [英] Using custom methods in linq to entities

问题描述

我的数据库中有一个Person表，该表具有NationalId字段. 有什么方法可以使用Ef code first和Linq to entities甚至使用Ef code first和Linq to entities来加载所有带有NationalId的人员，而不是将所有Person都加载到内存中?

I have a Person table in my database that has NationalId field. Is there any way to load all Persons with even NationalId, using Ef code first and Linq to entities, without loading all Persons to memory?

类似的东西

public  bool IsEven(int number)
{
   return number % 2 == 0;
}

var context = new MyContext();
var personsWithEvenNationalId = context.Persons
                                       .Where(x=> IsEven(x.NationalId))
                                       .ToList();

推荐答案

您必须进行内联检查

var personsWithEvenNationalId = context.Persons
                                       .Where(x=> x.NationalId%2 == 0)
                                       .ToList();

Linq to Entities基本上不知道如何将自定义方法转换为SQL.如果确实需要使用自定义方法，则必须使Persons枚举，然后使用自定义方法，即

Linq to Entities doesn't know how to translate your custom method into SQL basically. If you do need to use a custom method you would have to get Persons as enumerable and then use your custom method, i.e.

var personsWithEvenNationalId = context.Persons
                                       .AsEnumerable()
                                       .Where(x=> IsEven(x.NationalId))
                                       .ToList();

但这并不理想，因为它将加载所有Person，然后在IsEven上进行过滤

But that's hardly ideal as it would load all Persons and then filter on IsEven

考虑一下它，如果不想每次都内联编写，也可以为IQueryable<Person>创建扩展方法.在您构建Expression

Thinking about it you could also create an extension method for IQueryable<Person> if you don't want to have to write it inline every time. Something like this where you build an Expression

    public static IQueryable<Person> WhereEven(this IQueryable<Person> source, Expression<Func<Person, int>> property)
    {
        var expression = Expression.Equal(
            Expression.Modulo(
                property.Body,
                Expression.Constant(2)),
            Expression.Constant(0));

        var methodCallExpression = Expression.Call(typeof (Queryable),
            "where",
            new Type[] {source.ElementType},
            source.Expression,
            Expression.Lambda<Func<Person, bool>>(expression, property.Parameters));

        return source.Provider.CreateQuery<Person>(methodCallExpression);
    }

并使用它:

context.Persons.WhereEven(x => x.NationalId).ToList();

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