如何在实体框架中重用投影? [英] How to reuse projections in Entity Framework?
问题描述
我有一个使用实体框架获取数据的ASP.NET MVC应用程序.
I have an ASP.NET MVC application which uses Entity Framework to get data.
我需要先将实体转换为模型,然后再将其传递给视图.投影可能非常复杂,但要保持简单:
I need to transform Entites to Models before passing them to View. Projections can be very complex, but to keep it simple:
public static IQueryable<UserModel> ToModel(this IQueryable<User> users)
{
return from user in users
select new UserModel
{
Name = user.Name,
Email = user.Email,
};
}
这可以在像这样的控制器中使用:
This can be used in a controller like this:
return View(Repository.Users.ToModel().ToList());
很好.但是,如果我想在另一个投影中使用该投影怎么办?示例:
Very good. But what if I want to use this projection inside another one? Example:
public static IQueryable<BlogPostModel> ToModel(this IQueryable<BlogPost> blogs)
{
return from blogs in blogs
select new BlogPostModel
{
Title = blog.Title,
Authors = blog.Authors.AsQueryable().ToModel(), // (entities are POCOs)
// This does not work, because EF does not understand method ToModel().
};
}
(假设博客的作者人数可以超过一个,并且该博客的类型为User).
(let's suppose blog can have more then one author and it is of type User).
我可以以某种方式分离投影并在其他投影中重复使用吗?
Can I somehow separate the projections and reuse them inside another ones?
推荐答案
以下是实际工作的内容(在简单的测试应用程序中),它只能选择请求的字段:
Here's something that actually works (in a simple test application) to only select the requested fields:
namespace Entities
{
public class BlogPost
{
public virtual int Id { get; set; }
public virtual string Title { get; set; }
public virtual DateTime Created { get; set; }
public virtual ICollection<User> Authors { get; set; }
}
public class User
{
public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual string Email { get; set; }
public virtual byte[] Password { get; set; }
public virtual ICollection<BlogPost> BlogPosts { get; set; }
}
}
namespace Models
{
public class BlogPostModel
{
public string Title { get; set; }
public IEnumerable<UserModel> Authors { get; set; }
}
public class UserModel
{
public string Name { get; set; }
public string Email { get; set; }
}
public static class BlogPostModelExtensions
{
public static readonly Expression<Func<BlogPost, BlogPostModel>> ToModelConverterExpression =
p =>
new BlogPostModel
{
Title = p.Title,
Authors = p.Authors.AsQueryable().Select(UserModelExtensions.ToModelConverterExpression),
};
public static readonly Func<BlogPost, BlogPostModel> ToModelConverterFunction = ToModelConverterExpression.Compile();
public static IQueryable<BlogPostModel> ToModel(this IQueryable<BlogPost> blogPosts)
{
return blogPosts.Select(ToModelConverterExpression);
}
public static IEnumerable<BlogPostModel> ToModel(this IEnumerable<BlogPost> blogPosts)
{
return blogPosts.Select(ToModelConverterFunction);
}
}
public static class UserModelExtensions
{
public static readonly Expression<Func<User, UserModel>> ToModelConverterExpression =
u =>
new UserModel
{
Name = u.Name,
Email = u.Email,
};
public static readonly Func<User, UserModel> ToModelConverterFunction = ToModelConverterExpression.Compile();
public static IQueryable<UserModel> ToModel(this IQueryable<User> users)
{
return users.Select(ToModelConverterExpression);
}
public static IEnumerable<UserModel> ToModel(this IEnumerable<User> users)
{
return users.Select(ToModelConverterFunction);
}
}
}
要在不实际创建数据库的情况下对其进行测试:
To test it without actually creating a database:
var blogPostsQuery = (
from p in context.BlogPosts
where p.Title.StartsWith("a")
select p).ToModel();
Console.WriteLine(((ObjectQuery)blogPostQuery).ToTraceString());
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