Entity Framework, LINQ 查询

Question

如何循环遍历下面的项目并用值 (!) 错误：

Cannot implicitly convert type System.threading.task to System.Collections.Generic.IEnumerable<Respositories.AssignmentMasterData>

using (SUPEntities db = new SUPEntities())
{
    IEnumerable<AssignementMasterData> masterDatas = null;

    masterDatas = db.AssignementMasterDatas
                    .Where(m => DbFunctions.TruncateTime(m.CreatedDateTime) >= DbFunctions.TruncateTime(criteria.FilterStartDate)
                        && DbFunctions.TruncateTime(m.CreatedDateTime) <= DbFunctions.TruncateTime(criteria.FilterEndDate)
                        && (m.AssignmentNoteNumber == criteria.AssigmentNumber || criteria.AssignmentNumber == null)
                        && (m.BaseCourseId == criteria.courseId || criteria.CourseId == 0)
                        && (m.AccountNumber == criteria.AccountNumber || criteria.AccountNumber == null)
                        && (m.ReferenceNumber == criteria.ReferenceNumber || criteria.ReferenceNumber == null)
                        && (m.FacultyCode == criteria.FAcultyCode || criteria.FacultyCode == null)
                        && (m.Processed == criteria.Processed)
                        && (m.ClassNumber == criteria.ClassNumber || criteria.ClassNumber == null))
                    .ForEachAsync(t => t.AssignmentNoteIdentifiedClasses.Select(e => String.IsNullOrEmpty(e.Category)? "(!)": e.Category));
                
}

Answer 1

首先，关于错误信息：

您正试图将错误的类型分配给您的 masterDatas 变量。您将其声明为 IEnumerable<Respositories.AssignmentMasterData>，但最后一行的 ForEachAsync 将 return 为 Task，因此出现错误消息。

查看ForEachAsync签名：

public static System.Threading.Tasks.Task ForEachAsync (this System.Linq.IQueryable source, Action action);

其次。你想要 return 一个 IEnumerable<Respositories.AssignmentMasterData>

如果您可以满足于同步方法，您可以这样做：

您需要在某个时候将您的 IQueryable 转换为 IEnumerable。调用 AsEnumerable() 即可。然后你需要替换一些值。所以你需要使用 Select.

投影你的 collection

using (SUPEntities db = new SUPEntities())
{
    var masterDatas = db.AssignementMasterDatas
        .Where(m => DbFunctions.TruncateTime(m.CreatedDateTime) >= DbFunctions.TruncateTime(criteria.FilterStartDate)
            && DbFunctions.TruncateTime(m.CreatedDateTime) <= DbFunctions.TruncateTime(criteria.FilterEndDate)
            && (m.AssignmentNoteNumber == criteria.AssigmentNumber || criteria.AssignmentNumber == null)
            && (m.BaseCourseId == criteria.courseId || criteria.CourseId == 0)
            && (m.AccountNumber == criteria.AccountNumber || criteria.AccountNumber == null)
            && (m.ReferenceNumber == criteria.ReferenceNumber || criteria.ReferenceNumber == null)
            && (m.FacultyCode == criteria.FAcultyCode || criteria.FacultyCode == null)
            && (m.Processed == criteria.Processed)
            && (m.ClassNumber == criteria.ClassNumber || criteria.ClassNumber == null))
        .AsEnumerable()
        .Select(a =>
        {
            a.AssignmentNoteIdentifiedClasses = a.AssignmentNoteIdentifiedClasses
                .Select(e =>
                {
                    e.Category = string.IsNullOrWhiteSpace(e.Category) ? "(!)" : e.Category;
                    return e;
                })
                .ToList(); // Depending on the type of AssignmentNoteIdentifiedClasses, ToList() might be replaced.
            return a;
        });
    return masterDatas;
}

Answer 2

使用 .Include( m => m.AssignmentNoteIdentifiedClasses ) 在单个查询中引入相关数据，这比在 for 中加载每组相关 AssignmentNoteIdentifiedClasses 快得多 - 每行循环。
- This is also known as the "N+1 Problem" in ORMs.
您不需要使用 TruncateTime。
- 事实上，您不应该，因为那将意味着您的查询不可搜索。
- 避免 SQL 谓词中的函数。
- 相反，只需将 criteria.FilterStartDate 舍入到应用程序代码中的一天的开始，然后将其与 m => m.CreatedDateTime >= filterStart 进行正常比较。
- 同样，FilterEndDate 应该四舍五入，然后像这样进行比较：m => m.CreatedDateTime < filterEnd
  - 始终使用独占上限。它使一切，尤其是日期范围谓词，更容易处理。
您的 Where 中不需要内联 &&。请改用额外的单独 .Where() 子句。它们将作为单独的 AND 个条款添加到相同的（单个）WHERE 子句中。
我假设 EF Core 不够复杂，无法识别可选搜索谓词的“NULL-means-ignore”反模式，在这种情况下不要使用“ NULL-means-ignore" anti-pattern IN AN IQUERYABLE<T> PREDICATE!
- 这很糟糕，原因有很多：即因为查询执行计划是基于结构（“形状”）的 SQL 查询和 not 参数值，因此当某些甚至所有参数为 NULL 时，将使用非 NULL 参数的相同缓存执行计划 - 这是一个问题.另外，请务必阅读 parameter sniffing.
- 相反，通过使用 IQueryable<T> 的 Linq 扩展并重新分配给自身来构建您的查询。
  - 例如IQueryable<T> query = db.Etc; query = query.Where( e => etc );
  - 每个 .Where() 添加为一个 AND 条件。如果你想建立一个 OR 条件 then use PredicateBuilder.

DateTime filterStart   = criteria.FilterStartDate.Date;
DateTime filterEndExcl = criteria.FilterEndDate  .Date.AddDays(1);

using (SUPEntities db = new SUPEntities())
{
    IQueryable<AssignementMasterData> query = db.AssignementMasterDatas
        .Include( m => m.AssignmentNoteIdentifiedClasses )
        .Where( m => m.CreatedDateTime >= filterStart   )
        .Where( m => m.CreatedDateTime <  filterEndExcl ) // Exclusive upper-bound.
        .Where( m => m.Processed       == criteria.Processed )
        .Where( m => m.ClassNumber     == criteria.ClassNumber )
    ;

    if( criteria.AssigmentNumber != null )
    {
        query = query.Where( m => m.AssignmentNoteNumber == criteria.AssigmentNumber );
    }

    if( criteria.AccountNumber != null )
    {
        query = query.Where( m => m.AccountNumber == criteria.AccountNumber );
    }

    if( criteria.CourseId != null && criteria.CourseId.Value > 0 )
    {
        query = query.Where( m => m.BaseCourseId == criteria.CourseId );
    }

    if( criteria.ReferenceNumber != null )
    {
        query = query.Where( m => m.ReferenceNumber == criteria.ReferenceNumber );
    }

    if( criteria.FacultyCode != null )
    {
        query = query.Where( m => m.FacultyCode == criteria.FacultyCode );
    }

    if( criteria.ClassNumber != null )
    {
        query = query.Where( m => m.ClassNumber == criteria.ClassNumber );
    }

    List<AssignementMasterData> rows = await query.ToListAsync().ConfigureAwait(false);
    
    List<String> categories = rows
        .SelectMany( r => r.AssignmentNoteIdentifiedClasses )
        .Select( String.IsNullOrEmpty(e.Category)? "(!)": e.Category) )
        .ToList();

    return categories;
}

可以通过添加新的扩展方法来简化上述内容（确保使用 Expression<Func<...>> 而不仅仅是 Func<>，这样 EF 仍然可以解释查询：

public static class MyQueryableExtensions
{
    public static IQueryable<T> WhereIfNotNull<T,TValue>( this IQueryable<T> query, TValue? value, Expression<Func<T,Boolean>> predicate )
        where TValue : struct
    {
        if( value.HasValue && value.Value != default(TValue) )
        {
            return query.Where( predicate );
        }
        else
        {
            return query;
        }
    }
}

这样使用：

// `criteria` is now named `c` for brevity.

DateTime filterStart   = c.FilterStartDate.Date;
DateTime filterEndExcl = c.FilterEndDate  .Date.AddDays(1);

using (SUPEntities db = new SUPEntities())
{
    IQueryable<AssignementMasterData> query = db.AssignementMasterDatas
        .Include( m => m.AssignmentNoteIdentifiedClasses )
        .Where( m => m.CreatedDateTime >= filterStart    )
        .Where( m => m.CreatedDateTime <  filterEndExcl  ) // Exclusive upper-bound.
        .Where( m => m.Processed       == c.Processed    )
        .Where( m => m.ClassNumber     == c.ClassNumber  )
        .WhereIfNotNull( c.AssigmentNumber, m => m.AssignmentNoteNumber == c.AssigmentNumber )
        .WhereIfNotNull( c.AccountNumber  , m => m.AccountNumber        == c.AccountNumber   )
        .WhereIfNotNull( c.CourseId       , m => m.BaseCourseId         == c.CourseId        )
        .WhereIfNotNull( c.ReferenceNumber, m => m.ReferenceNumberr     == c.ReferenceNumber )
        .WhereIfNotNull( c.FacultyCode    , m => m.FacultyCoder         == c.FacultyCode     )
        .WhereIfNotNull( c.ClassNumber    , m => m.ClassNumber          == c.ClassNumber     )
   ;

    List<AssignementMasterData> rows = await query.ToListAsync().ConfigureAwait(false);
    
    List<String> categories = rows
        .SelectMany( r => r.AssignmentNoteIdentifiedClasses )
        .Select( String.IsNullOrEmpty(e.Category)? "(!)": e.Category) )
        .ToList();

    return categories;
}

Entity Framework, LINQ 查询

Entity Framework, LINQ Query

c#

linq

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