EF Core linq和条件包含然后包含问题 [英] EF Core linq and conditional include and theninclude problem

问题描述

尝试获取具有多个级别的对象时，我在获取结果时遇到问题。
这是我要大致执行的操作：

I am having a problem with getting a result when trying to get objects with multiple levels. This is what I am trying to do roughly:

_context.Investors.Where(s => s.Id == userId)
    .Include(c => c.Coins) //only want this if some kind of flag is given by the user.
    .ThenInclude(ct => ct.CoinType)
    .Include(c => c.Bricks) //only want this if some kind of flag is given by the user.

基本上，我收到很多标志，指示是否应包括对象的一部分。
我差不多可以用了。像这样：

Essentially I am getting a lot of flags indicating if I should include parts of the object. I got it almost to work. Like this:

_context.Investors.Where(s => s.Id == userId)
    .Select(i => new
    {
        i,
        Bricks = (details & GapiInvestorFlags.Bricks) != GapiInvestorFlags.Bricks ? null : i.Bricks,
        Offers = (details & GapiInvestorFlags.Offers) != GapiInvestorFlags.Offers ? null : i.Offers,
        Coins = (details & GapiInvestorFlags.Coins) != GapiInvestorFlags.Coins ? null : i.Coins,
        CoinTransactions = (details & GapiInvestorFlags.CoinTransactions) != GapiInvestorFlags.CoinTransactions ? null : i.CoinTransactions,
        OfferTransactions = (details & GapiInvestorFlags.OfferTransactions) != GapiInvestorFlags.OfferTransactions ? null : i.OfferTransactions,
        BuyTransactions = (details & GapiInvestorFlags.BuyTransactions) != GapiInvestorFlags.BuyTransactions ? null : i.BuyTransactions,
        SellTransactions = (details & GapiInvestorFlags.SellTransactions) != GapiInvestorFlags.SellTransactions ? null : i.SellTransactions
    }).AsEnumerable()
    .Select(e => e.i).FirstOrDefault();

这可以工作，除了Coins部分也有硬币类型，所以我也需要包括它。但是，当我添加代码时，整个部分将停止工作。

This works except that the Coins section also has a cointype in it so I need to include it too. But when I add my code, the whole section stops working.

这是我尝试过的操作：

_context.Investors.Where(s => s.Id == userId)
    .Include(c => c.Coins)
    .ThenInclude(ct => ct.CoinType)
    .Select(i => new
    {
        i,
        Bricks = (details & GapiInvestorFlags.Bricks) != GapiInvestorFlags.Bricks ? null : i.Bricks,
        Offers = (details & GapiInvestorFlags.Offers) != GapiInvestorFlags.Offers ? null : i.Offers,
        Coins = (details & GapiInvestorFlags.Coins) != GapiInvestorFlags.Coins ? null : i.Coins.Select(c => new { c, c.CoinType }).ToList(),
        CoinTransactions = (details & GapiInvestorFlags.CoinTransactions) != GapiInvestorFlags.CoinTransactions ? null : i.CoinTransactions,
        OfferTransactions = (details & GapiInvestorFlags.OfferTransactions) != GapiInvestorFlags.OfferTransactions ? null : i.OfferTransactions,
        BuyTransactions = (details & GapiInvestorFlags.BuyTransactions) != GapiInvestorFlags.BuyTransactions ? null : i.BuyTransactions,
        SellTransactions = (details & GapiInvestorFlags.SellTransactions) != GapiInvestorFlags.SellTransactions ? null : i.SellTransactions
    }).AsEnumerable()
    .Select(e => e.i).FirstOrDefault();

我真的不知道为什么它不起作用。

I really can't tell why it does not work.

基本上是在我更改时：

i.Coins

至

i.Coins.Select(c => new { c, c.CoinType }).ToList()

它将停止工作。

推荐答案

您使用的技术并不是真正的显式加载（ Include / ThenInclude ），但是基于投影和EF Core导航属性修复的技巧，所以我不能说为什么它停止工作。 EF Core仍然处理预测并包含不同的内容，因此它可能是当前处理中的缺陷。

The technique you are using is not really explicit loading (Include / ThenInclude), but trick based on projection and EF Core navigation property fix-up, so I can't say why it stops working. EF Core still processes projections and includes differently, so it might be a defect in the current processing.

在根查询级别实施条件包含相对容易。请注意， Include 方法从 IQueryable< TEntity> （已定义）开始，并返回 IIncludableQueryable< TEntity，TPreviousProperty>> 也是 IQueryable< TEntity> 。这意味着您可以保留 IQueryable< T> 查询变量，并应用类似于链式 Where 运算符的条件转换。

Implementing conditional include at the root query level is relatively easy. Note that the Include method starts from (is defined for) IQueryable<TEntity> and the returned IIncludableQueryable<TEntity, TPreviousProperty>> is also IQueryable<TEntity>. Which means you can keep IQueryable<T> query variable and apply conditional transformations similar to chained Where operators.

要使其更容易，您可以创建一个自定义帮助程序扩展方法，如下所示：

To make that easier, you could create a custom helper extension method like this:

public static IQueryable<T> If<T>(
    this IQueryable<T> source,
    bool condition,
    Func<IQueryable<T>, IQueryable<T>> transform
)
{ 
    return condition? transform(source) : source;
}

并像这样使用它：

_context.Investors.Where(s => s.Id == userId)
    .If(flagCoins, q => q.Include(e => e.Coins)
        .ThenInclude(e => e.CoinType))
    .If(flagBricks, q => q.Include(e => e.Bricks))

如果您需要类似的嵌套级别（ ThenInclude ），然后添加以下2种扩展方法：

If you need something similar for the nested levels (ThenInclude), then add the following 2 extension methods:

public static IQueryable<T> If<T, P>(
    this IIncludableQueryable<T, P> source,
    bool condition,
    Func<IIncludableQueryable<T, P>, IQueryable<T>> transform
)
    where T : class
{
    return condition ? transform(source) : source;
}

public static IQueryable<T> If<T, P>(
    this IIncludableQueryable<T, IEnumerable<P>> source,
    bool condition,
    Func<IIncludableQueryable<T, IEnumerable<P>>, IQueryable<T>> transform
)
    where T : class
{
    return condition ? transform(source) : source;
}

这将允许您使用类似这样的东西：

which will allow you to use something like this:

_context.Investors.Where(s => s.Id == userId)
    .If(flagCoins, q => q.Include(e => e.Coins)
        .If(flagCoinType, q2 => q2.ThenInclude(e => e.CoinType)))
    .If(flagBricks, q => q.Include(e => e.Bricks))

这篇关于EF Core linq和条件包含然后包含问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！