How do I use Linq and Entity Framework to join two jointables?

问题

I have a very normalized database, and I'm trying to join two join tables together.

My goal is to only show documents that the user has permission to. I'm using Entity Framework, and have several foreign keys set up for the following tables:

Relationships (Foreign Keys)

Users  --------------------------------------------- 
                                                    |
                          UserGroupMembership    (UserID, GroupID)
                                                              |  
                                                              |
Groups ----- -------------------------------------------------|
    |
    |
    |---------------------------------------------------------|
                                                              | 
                                                              | 
XDocuments               XDocumentSecurity   (DocumentID, GroupID)
       |                                           |
       ---------------------------------------------

Table Definition

public partial class Users : EntityObject  
{
    public int UserID {get;set;} //PK
    public string UserDisplayName {get;set;}  
    public DateTime CreateDate {get;set;} 
    public DateTime LoginDate {get;set;}  
} 

public partial class Groups : EntityObject  
{
    public int GroupID {get;set;} //PK
    public string GroupDisplayName {get;set;}  
    public DateTime CreateDate {get;set;} 
    public DateTime LoginDate {get;set;}  
} 


public partial class UserGroupMembership: EntityObject  // JoinTable
{
   public int UserID {get;set;} //PK
   public int GroupID {get;set;} //PK

   // Not sure if this extra columns below causes an issue
   public bool CanView {get;set;}
   public bool CanDelete {get;set;} 
   public bool CanUpdate {get;set;} 
   public DateTime CreateDate {get;set;}   
}

public partial class XDocumentSecurity : EntityObject // JoinTable
{
    public int DocumentID {get;set;} //FK
    public int GroupID {get;set;} //FK

     public DateTime CreateDate {get;set;}   // Not sure if this extra column causes an issue  
 } 

public partial class XDocuments : EntityObject  
{
    public int DocumentID {get;set;} //PK
    public string URL {get;set;}  
    public DateTime CreateDate {get;set;} 
} 

I've heard of many stories about how Linq to EF alters the SQL query in a way that is sub-optimal for performance.

Here is is the .Union operator sample that seems most applicable to what I'm doing. I could simply get a list of groups the current user is a member of and issue a modified version of this query:

public void Linq50() 
{ 
    int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 }; 
    int[] numbersB = { 1, 3, 5, 7, 8 }; 

    var commonNumbers = numbersA.Intersect(numbersB); 

    Console.WriteLine("Common numbers shared by both arrays:"); 
    foreach (var n in commonNumbers) 
    { 
        Console.WriteLine(n); 
    } 
}

Question

• How do I view the SQL query that EF generates?
• What is a better way to approach this problem (if any)

回答1:

You can also view the SQL generated by EF 4.1 using either

• SQL Profiler
• LINQPad

Although, at this time, you need to use the LINQPad beta for EF 4.1.

Regarding your second question, I believe your query would translate fine. Using LINQPad to check the SQL, the following query

var a1 = Addresses.Where(a => a.City.ToUpper().EndsWith("L")).Select(a => a.AddressID);
var a2 = Addresses.Where(a => a.City.ToUpper().StartsWith("B")).Select(a => a.AddressID);

var x1 = a1.Intersect(a2);

translates to

SELECT 
[Intersect1].[AddressID] AS [C1]
FROM  (SELECT 
    [Extent1].[AddressID] AS [AddressID]
    FROM [Person].[Address] AS [Extent1]
    WHERE UPPER([Extent1].[City]) LIKE N'%L'
INTERSECT
    SELECT 
    [Extent2].[AddressID] AS [AddressID]
    FROM [Person].[Address] AS [Extent2]
    WHERE UPPER([Extent2].[City]) LIKE N'B%') AS [Intersect1]

---

I think @Slauma's recommendation to use navigation proeprties is the way to go if your model supports it.

Still, get LINQPad - you won't regret it :)

回答2:

If you have navigation properties for all your keys and foreign keys an alternative query without an Intersect would be:

var query = context.XDocuments
    .Where(d => d.Groups.Any(g => g.Users.Any(u => u.UserID == givenUserId)));

("Filter all documents which are in at least one group which has at least one user with the key = givenUserId")

I don't know if this will be better with respect to performance.

In EF 4.1 you can inspect the generated SQL simply by:

var sql = query.ToString();

Edit

My understanding how your model would look is the following:

Three entities with corresponding tables:

public class User
{
    public int UserID { get; set; }
    public ICollection<Group> Groups { get; set; }
}

public class Group
{
    public int GroupID { get; set; }
    public ICollection<User> Users { get; set; }
    public ICollection<XDocument> Documents { get; set; }
}

public class XDocument
{
    public int DocumentID { get; set; }
    public ICollection<Group> Groups { get; set; }
}

And between User and Group a many-to-many relationship and between Group and XDocument as well:

modelBuilder.Entity<User>()
    .HasMany(u => u.Groups)
    .WithMany(g => g.Users)
    .Map(c =>
    {
        c.MapLeftKey("UserID");
        c.MapRightKey("GroupID");
        c.ToTable("UserGroupMembership");  // join table name, no entity
    });

modelBuilder.Entity<XDocument>()
    .HasMany(d => d.Groups)
    .WithMany(g => g.Documents)
    .Map(c =>
    {
        c.MapLeftKey("DocumentID");
        c.MapRightKey("GroupID");
        c.ToTable("XDocumentSecurity");  // join table name, no entity
    });

In this model and mapping the query described above should be possible. There is no need to access the join tables directly (and you can't actually access them via LINQ to Entities, EF manages those tables internally).

来源：https://stackoverflow.com/questions/8016773/how-do-i-use-linq-and-entity-framework-to-join-two-jointables