Select 带有子查询的查询 return List<string>

Question

我有两个 table：产品和产品图片

我正在尝试编写一个查询来返回产品，并为每个产品关联一个图像名称列表，但不断收到错误消息（如下所示）。

实体如下所示：

public class ProductDto
{
    public int Id { get; set; }

    public string Name { get; set; }

    public long ProductNumber { get; set; }

    public decimal? Size { get; set; }

    public string SizeMeasurement { get; set; }

    public decimal? Pack { get; set; }

    public List<string> PhotoFileNames { get; set; }
}

此处显示出错的查询：

var query = from p in _productRepository.GetAll()
        let PhotoFileNames =  _productPhotoMapRepository.GetAll().DefaultIfEmpty()
            .Where(e => e.ProductId == p.Id).Select(e => e.FileName).ToList()
    where
    (string.IsNullOrWhiteSpace(input.Filter) || p.Name.Contains(input.Filter))
    select new
    {
        p.Id,
        p.ProductNumber,
        p.Name,
        p.Size,
        p.SizeMeasurement,
        p.Pack,
//I've also tried doing this but get same error:
//PhotoFileNames = _productPhotoMapRepository.GetAll().DefaultIfEmpty()
//                     .Where(e => e.ProductId == p.Id).Select(e => e.FileName).ToList()
        PhotoFileNames
    };

    var productList = await query.ToListAsync();

我收到的错误是：

System.ArgumentException: Expression of type 'System.Collections.Generic.IAsyncEnumerable`1[System.String]' cannot be used for parameter of type 'System.Collections.Generic.IEnumerable`1[System.String]' of method 'System.Collections.Generic.List`1[System.String] ToList[String](System.Collections.Generic.IEnumerable`1[System.String])'
Parameter name: arg0
   at System.Dynamic.Utils.ExpressionUtils.ValidateOneArgument(MethodBase method, ExpressionType nodeKind, Expression arguments, ParameterInfo pi, String methodParamName, String argumentParamName, Int32 index)
   at System.Linq.Expressions.Expression.Call(MethodInfo method, Expression arg0)
   at System.Linq.Expressions.MethodCallExpression1.Rewrite(Expression instance, IReadOnlyList`1 args)
   at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.RelationalEntityQueryableExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at System.Linq.Expressions.ExpressionVisitor.VisitAndConvert[T](ReadOnlyCollection`1 nodes, String callerName)
   at Remotion.Linq.Parsing.RelinqExpressionVisitor.VisitNew(NewExpression expression)
   at System.Linq.Expressions.NewExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at System.Dynamic.Utils.ExpressionVisitorUtils.VisitArguments(ExpressionVisitor visitor, IArgumentProvider nodes)
   at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.RelationalEntityQueryableExpressionVisitor.VisitMethodCall(MethodCallExpression node)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)

出于性能原因，我试图避免使用 for 循环。如果我完全省略 PhotoFileNames，产品 return 没有任何问题。

获得每个产品和存储在不同 sql table 中的所有图像文件名的预期结果的正确方法是什么？

编辑：添加了数据库上下文和模型类

public class Product
    {
        public int Id { get; set; } //Aka productId

        public string Name { get; set; }

        public long ProductNumber { get; set; }

        public decimal? Size { get; set; }

        public string SizeMeasurement { get; set; }

        public decimal? Pack { get; set; }
    }

       public class ProductPhotoMap 
        {
           public virtual long ProductPhotoId {get;set;}

            public virtual int DisplayOrder { get; set; }

            public virtual bool IsDefault { get; set; }     

            public virtual int ProductId { get; set; }
            public Product Product { get; set; }

            public virtual string FileName { get; set; }

        }

在我的上下文中：

    public virtual DbSet<Product> Products { get; set; }

    public virtual DbSet<ProductPhotoMap> ProductPhotoMappings { get; set; }

Answer 1

.ToListAsync() 不 return 一个 List<string>。您可以更新属性，或非异步使用 .ToList()。

Answer 2

您的整个查询完全混乱。所以很难猜到你真正想要的是什么。但我假设您可能想要以下内容：

所以首先更新你的 Product class 如下：

public class Product
{
    public int Id { get; set; } //Aka productId

    public string Name { get; set; }

    public long ProductNumber { get; set; }

    public decimal? Size { get; set; }

    public string SizeMeasurement { get; set; }

    public decimal? Pack { get; set; }


    public ICollection<ProductPhotoMap> ProductPhotoMaps {get; set;}
}

然后写查询如下：

var query = _productRepository.GetAll().Select(p => new ProductDto
       {
        Id = p.Id,
        ProductNumber = p.ProductNumber,
        Name = p.Name,
        Size = p.Size,
        SizeMeasurement = p.SizeMeasurement,
        Pack = p.Pack,
        PhotoFileNames = p.ProductPhotoMaps.Select(pp => pp.FileName).ToList()
    });

If(!string.IsNullOrWhiteSpace(input.Filter))
{
   query.Where(p => p.Name.Contains(input.Filter))
}

var productList = await query.ToListAsync();

Select 带有子查询的查询 return List<string>

Select query with subquery that can return List<string>

linq

linq-to-entities

entity-framework-core

asp.net-core