ASP.NET MVC和放大器; EF4实体框架 - 是否有使用实体VS只获取我需要的字段的任何性能问题？ [英] ASP.NET MVC & EF4 Entity Framework - Are there any performance concerns in using the entities vs retrieving only the fields i need?

问题描述

假设我们有3个表，用户，产品，采购。
存在需要以显示由用户做出的购买的图。

Lets say we have 3 tables, Users, Products, Purchases. There is a view that needs to display the purchases made by a user.

我可以查找这样做所需的数据：

I could lookup the data required by doing:

from p in DBSet<Purchases>.Include("User").Include("Product") select p;

不过，我很担心，这可能会影响性能，因为它会检索完整的对象。
另外，我只选择我需要的字段：

However, I am concern that this may have a performance impact because it will retrieve the full objects. Alternatively, I could select only the fields i need:

from p in DBSet<Purchases>.Include("User").Include("Product") select new SimplePurchaseInfo() { UserName = p.User.name, Userid = p.User.Id, ProductName = p.Product.Name ... etc };

所以我的问题是：
请告诉我这样做的最佳做法？

So my question is: Whats the best practice in doing this?

==编辑

感谢所有的答复。

【问题1】：我想知道所有的观点是否应该与平坦的ViewModels工作非常具体的数据为这一观点，还是应该的ViewModels包含实体对象

[QUESTION 1]: I want to know whether all views should work with flat ViewModels with very specific data for that view, or should the ViewModels contain the entity objects.

真实的例子：网友评论产品

Real example: User reviews Products

var query = from dr in productRepository.FindAllReviews()
            where dr.User.UserId = 'userid'
            select dr;
string sql = ((ObjectQuery)query).ToTraceString();

SELECT [Extent1].[ProductId] AS [ProductId], 
       [Extent1].[Comment] AS [Comment], 
       [Extent1].[CreatedTime] AS [CreatedTime], 
       [Extent1].[Id] AS [Id], 
       [Extent1].[Rating] AS [Rating], 
       [Extent1].[UserId] AS [UserId], 
       [Extent3].[CreatedTime] AS [CreatedTime1], 
       [Extent3].[CreatorId] AS [CreatorId], 
       [Extent3].[Description] AS [Description], 
       [Extent3].[Id] AS [Id1], 
       [Extent3].[Name] AS [Name], 
       [Extent3].[Price] AS [Price], 
       [Extent3].[Rating] AS [Rating1], 
       [Extent3].[ShopId] AS [ShopId], 
       [Extent3].[Thumbnail] AS [Thumbnail], 
       [Extent3].[Creator_UserId] AS [Creator_UserId], 
       [Extent4].[Comment] AS [Comment1], 
       [Extent4].[DateCreated] AS [DateCreated], 
       [Extent4].[DateLastActivity] AS [DateLastActivity], 
       [Extent4].[DateLastLogin] AS [DateLastLogin], 
       [Extent4].[DateLastPasswordChange] AS [DateLastPasswordChange], 
       [Extent4].[Email] AS [Email], 
       [Extent4].[Enabled] AS [Enabled], 
       [Extent4].[PasswordHash] AS [PasswordHash], 
       [Extent4].[PasswordSalt] AS [PasswordSalt], 
       [Extent4].[ScreenName] AS [ScreenName], 
       [Extent4].[Thumbnail] AS [Thumbnail1], 
       [Extent4].[UserId] AS [UserId1], 
       [Extent4].[UserName] AS [UserName]
       FROM    [ProductReviews] AS [Extent1]
       INNER JOIN [Users] AS [Extent2] ON [Extent1].[UserId] = [Extent2].[UserId]
       LEFT OUTER JOIN [Products] AS [Extent3] ON [Extent1].[ProductId] = [Extent3].[Id]
       LEFT OUTER JOIN [Users] AS [Extent4] ON [Extent1].[UserId] = [Extent4].[UserId]
       WHERE N'615005822' = [Extent2].[UserId]

或

from d in productRepository.FindAllProducts()
from dr in d.ProductReviews
where dr.User.UserId == 'userid'
orderby dr.CreatedTime
select new ProductReviewInfo()
       {
           product = new SimpleProductInfo() { Id = d.Id, Name = d.Name, Thumbnail = d.Thumbnail, Rating = d.Rating },
           Rating = dr.Rating,
           Comment = dr.Comment,
           UserId = dr.UserId,
           UserScreenName = dr.User.ScreenName,
           UserThumbnail = dr.User.Thumbnail,
           CreateTime = dr.CreatedTime
       };

SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[Name] AS [Name], 
[Extent1].[Thumbnail] AS [Thumbnail], 
[Extent1].[Rating] AS [Rating], 
[Extent2].[Rating] AS [Rating1], 
[Extent2].[Comment] AS [Comment], 
[Extent2].[UserId] AS [UserId], 
[Extent4].[ScreenName] AS [ScreenName], 
[Extent4].[Thumbnail] AS [Thumbnail1], 
[Extent2].[CreatedTime] AS [CreatedTime]
FROM    [Products] AS [Extent1]
INNER JOIN [ProductReviews] AS [Extent2] ON [Extent1].[Id] = [Extent2].[ProductId]
INNER JOIN [Users] AS [Extent3] ON [Extent2].[UserId] = [Extent3].[UserId]
LEFT OUTER JOIN [Users] AS [Extent4] ON [Extent2].[UserId] = [Extent4].[UserId]
WHERE N'userid' = [Extent3].[UserId]
ORDER BY [Extent2].[CreatedTime] ASC

[问题2]：请告诉我的丑外连接

[QUESTION 2]: Whats with the ugly outer joins?

推荐答案

<击>我觉得第二个查询会抛出异常，因为你不能映射结果与Linq到实体映射的.NET类型。你必须返回annonymous类型和它的LINQ到对象映射到你的对象，或者您必须使用一些先进的理念，为预测 - QueryView或DefiningQuery（在ESQL预测）（自定义SQL查询映射到新的只读实体）

通常更关心的是你的实体设计。如果选择单一的小实体，它不是一个很大的区别加载它，而不是所有的投影。如果您选择的实体名单，你应该考虑的预测 - expecially如果表中包含像为nvarchar（最大）或varbinar（最大），这不是你的结果需要的列！

Generally it is more about design of your entities. If you select single small entity it is not a big difference to load it all instead of projection. If you are selecting list of entities you should consider projections - expecially if tables contains columns like nvarchar(max) or varbinar(max) which are not needed in your result!

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