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仅选择具有Linq(EF核心)的特定字段

entity-framework-core .net-core c#

Paolo Tedesco · 技术社区 · 7 年前

我有一个 DbContext 我希望运行一个查询,只返回特定列,以避免获取所有数据。
IQueryable 原始类型,即不构造匿名类型。

// Install-Package Microsoft.AspNetCore.All
// Install-Package Microsoft.EntityFrameworkCore

using Microsoft.EntityFrameworkCore;
using System;
using System.Linq;

public class Person {
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class TestContext : DbContext {
    public virtual DbSet<Person> Persons { get; set; }
    public TestContext(DbContextOptions<TestContext> options) : base(options) {
    }
}

class Program {
    static void Main(string[] args) {

        var builder = new DbContextOptionsBuilder<TestContext>();
        builder.UseInMemoryDatabase(Guid.NewGuid().ToString());
        var context = new TestContext(builder.Options);

        context.Persons.Add(new Person { FirstName = "John", LastName = "Doe" });
        context.SaveChanges();

        // How can I express this selecting columns with a set of strings? 
        IQueryable<Person> query = from p in context.Persons select new Person { FirstName = p.FirstName };
    }
}

我想采用类似于此方法的方法:

static IQueryable<Person> GetPersons(TestContext context, params string[] fieldsToSelect) {
    // ...
}

我有办法做到这一点吗?

3 回复 | 直到 7 年前

Ivan Stoev 7 年前

因为您正在投影(选择)类型的成员 T 同类型 ,所需的 Expression<Func<T, T>> 可以相对容易地用 Expression 类方法,如下所示:

public static partial class QueryableExtensions
{
    public static IQueryable<T> SelectMembers<T>(this IQueryable<T> source, params string[] memberNames)
    {
        var parameter = Expression.Parameter(typeof(T), "e");
        var bindings = memberNames
            .Select(name => Expression.PropertyOrField(parameter, name))
            .Select(member => Expression.Bind(member.Member, member));
        var body = Expression.MemberInit(Expression.New(typeof(T)), bindings);
        var selector = Expression.Lambda<Func<T, T>>(body, parameter);
        return source.Select(selector);
    }
}

Expression.MemberInit new T { Member1 = x.Member1, Member2 = x.Member2, ... } C#构造。

return context.Set<Person>().SelectMembers(fieldsToSelect);

Janus Pienaar 7 年前

这可以通过使用 Dynamic Linq .

对于.Net核心- System.Linq.Dynamic.Core

使用动态Linq,您可以将SELECT和WHERE作为字符串传递。

使用您的示例,您可以执行以下操作:

IQueryable<Person> query = context.Persons
                        .Select("new Person { FirstName = p.FirstName }");

ornic 5 年前

answer 属于 Ivan 我制作了原始版本的缓存函数,以消除使用反射对我们造成的损失。它允许将这一通行费从秒到 微型的 重复请求的秒数(例如,DbAccess API的典型情况)。

public static class QueryableExtensions
{
    public static IQueryable<T> SelectMembers<T>(this IQueryable<T> source, IEnumerable<string> memberNames)
    {
        var result = QueryableGenericExtensions<T>.SelectMembers(source, memberNames);
        return result;
    }
}


public static class QueryableGenericExtensions<T>
{
    private static readonly ConcurrentDictionary<string, ParameterExpression> _parameters = new();
    private static readonly ConcurrentDictionary<string, MemberAssignment> _bindings = new();
    private static readonly ConcurrentDictionary<string, Expression<Func<T, T>>> _selectors = new();

    public static IQueryable<T> SelectMembers(IQueryable<T> source, IEnumerable<string> memberNames)
    {
        var parameterName = typeof(T).FullName;

        var requestName = $"{parameterName}:{string.Join(",", memberNames.OrderBy(x => x))}";
        if (!_selectors.TryGetValue(requestName, out var selector))
        {
            if (!_parameters.TryGetValue(parameterName, out var parameter))
            {
                parameter = Expression.Parameter(typeof(T), typeof(T).Name.ToLowerInvariant());

                _ = _parameters.TryAdd(parameterName, parameter);
            }

            var bindings = memberNames
                .Select(name =>
                {
                    var memberName = $"{parameterName}:{name}";
                    if (!_bindings.TryGetValue(memberName, out var binding))
                    {
                        var member = Expression.PropertyOrField(parameter, name);
                        binding = Expression.Bind(member.Member, member);

                        _ = _bindings.TryAdd(memberName, binding);
                    }
                    return binding;
                });

            var body = Expression.MemberInit(Expression.New(typeof(T)), bindings);
            selector = Expression.Lambda<Func<T, T>>(body, parameter);

            _selectors.TryAdd(requestName, selector);
        }

        return source.Select(selector);
    }
}

使用相同参数连续运行后的结果示例(请注意,这是纳秒):

SelectMembers time ... 3092214 ns
SelectMembers time ... 145724 ns
SelectMembers time ... 38613 ns
SelectMembers time ... 1969 ns

Bijay Koirala 7 年前

请尝试以下代码:

string fieldsToSelect = "new Person { FirstName = p.FirstName }"; //Pass this as parameter.

public static IQueryable<Person> GetPersons(TestContext context, string fieldsToSelect) 
{
    IQueryable<Person> query = context.Persons.Select(fieldsToSelect);
}

Ajeesh Joshy 6 年前

我可以用这个包裹做这件事 https://github.com/StefH/System.Linq.Dynamic.Core 太容易了。

使用名称空间, using System.Linq.Dynamic.Core;

//var selectQuery = "new(Name, Id, PresentDetails.RollNo)";

var selectQuery = "new(Name, Id, PresentDetails.GuardianDetails.Name as GuardianName)";

var students = dbContext.Students
    .Include(s => s.PresentDetails)
    .Include(s => s.PresentDetails.GuardianDetails)
    .Where(s => s.StudentStatus == "Admitted")
    .Select(selectQuery);

Tobias Lie-Atjam 4 年前

var students = dbContext.Students
    .Include(s => s.PresentDetails)
    .Where(s => s.StudentStatus == "Admitted")
    .Select(p => new Person() 
                       { 
                           Id = p.Id, 
                           Name = p.Name
                       });