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linq - 尝试获取计数时 Linq 中的子查询

问题描述

我试图让一些子查询在我的电话中工作。我试图让这个调用一次访问数据库，但我一生无法解决如何。查询在 GoodSections 部分中断。我尝试了许多不同的方法来做到这一点。我不断收到此消息：

无法翻译。以可翻译的形式重写查询，或通过插入对 AsEnumerable()、AsAsyncEnumerable()、ToList() 或 ToListAsync() 的调用显式切换到客户端评估

有人能帮我吗？

var test = context.UserAssessments.Include(n => n.Assessment).Include(n => n.UserSections).ThenInclude(userSection => userSection.Section)
                .OrderBy(n => n.StartDateTime);

            MyAssessments = await test.Select(assessment => new MyAssessmentVM()
            {
                Assessment = assessment.Assessment.Name,
                CompletedDateTime = assessment.CompletedDateTime,
                StartedDateTime = assessment.StartDateTime,
                UserAssessmentID = assessment.ID,
                GoodSections = assessment.UserSections.Where(userSection => userSection.Section.SectionType != SectionTypeEnum.Reading)
                            .Count(n => n.Percentage < n.Section.ReadinessRangeHigh && n.Percentage > n.Section.ReadinessRangeLow)
            }).ToListAsync();

标签： linq.net-core

解决方案

你的代码：

 GoodSections = assessment.UserSections
    .Where(userSection => userSection.Section.SectionType != SectionTypeEnum.Reading)

    // End of Where!

    .Count(n => n.Percentage < n.Section.ReadinessRangeHigh 
             && n.Percentage > n.Section.ReadinessRangeLow)

显然，每个 Assesment 都有一个由零个或多个 UserSection 组成的序列。在我看来，每个 UserSection 都有一个百分比和一个部分。

您使用 Include 来访问此 Section 的值，因此我假设 Section 在不同的表中，具有一对多关系：每个 Section 是零个或多个 UserSection 的部分；每个 UserSection 都属于确切的一个 Section，即外键所指的 Section。

首先，尝试简化您的计数，如果这没有帮助，请考虑使用 GroupJoin。

GoodSections = assessment.UserSections
    .Where(userSection => userSection.Section.SectionType != SectionTypeEnum.Reading
                       && userSection.Percentage < userSection.Section.ReadinessRangeHigh 
                       && userSection.Percentage > userSection.Section.ReadinessRangeLow)
    .Count(),

自己做 GroupJoin：

var test = dbContext.UserAssessments.GroupJoin(
    dbContext.UserSections,

    userAssessment => userAssesment.Id      // from every Assessment take the primary key
    userSection => userSection.AssesmentId, // from every UserSection take the foreign key

    // parameter resultSelector: from every UserAssesment, with all its UserSections
    // make one new
    (userAssessment, userSectionsOfThisAssessment) => new
    {
        UserAssessmentID = userAssessment.ID,
        StartedDateTime = userAssessment.StartDateTime,
        CompletedDateTime = userAssessment.CompletedDateTime,

        // To get the name, we need to get the Assesment that my foreign key refers to
        AssessmentName = dbContext.Assessments
             .Where(assessment => assessment.AssessmentId == userAssesment.Id)
             .Select(assessment => assessment.Name)
             .FirstOrDefault(),

        GoodSections = ... // TODO
    });

我不确定，但在我看来，Sections 和 UserSections 之间存在一对多的关系：每个 Section 都有零个或多个 UserSections；每个 UserSection 只属于一个 Section，即外键所指的 Section。

因此，对于每个 userSectionOfThisAssessment，我们需要获取外键引用的 Section：标准内部连接

GoodSections = userSectionsOfThisAssessment.Join(
    dbContext.Sections

    userSection => userSection.SectionId,    // take the foreign key to the section
    section => section.Id,                   // take the sections's primary key

    (userSection, section) => new
    {
        SectionType = section.SectionType,

        Percentage = userSection.Percentage,
        MaxPercentage = section.ReadinessRangeHigh,
        MinPercentage = section.ReadinessRangeLow,
    })
    .Where(joinResult => joinResult.SectionType != SectionTypeEnum.Reading
                      && joinResult.Percentage < MaxPercentage
                      && joinResult.Percentage > MinPercentage)
    .Count(),

对于 GoodSection，我们需要userSectionsOfThisAssessment与所有 Section 进行 GroupJoin。我不确定这是一对多的关系，还是一个男人

        MyAssessments = await test.Select(assessment => new MyAssessmentVM()
        {
            Assessment = assessment.Assessment.Name,
            CompletedDateTime = assessment.CompletedDateTime,
            StartedDateTime = assessment.StartDateTime,
            UserAssessmentID = assessment.ID,
            GoodSections = assessment.UserSections.Where(userSection => userSection.Section.SectionType != SectionTypeEnum.Reading)
                        .Count(n => n.Percentage < n.Section.ReadinessRangeHigh && n.Percentage > n.Section.ReadinessRangeLow)
        }).ToListAsync();

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