How to Implement Many-to-Many Relationships with Custom Join Tables in Entity Framework?

Create multiple -to -many relationships with a custom connection table

In the physical framework, most of the relationships usually involve a hidden connection table of an internal management. However, in some cases, you may need a custom connection table with additional attributes.

The limitations of more relationships

Unfortunately, the use of Fluent API cannot create a multi -pair relationship with a custom connection table. EF requires a hidden, internal management connection table to achieve multiple relationships.

Alternative method

On the contrary, to use a custom connection table with additional attributes, you can create two separate one -to -multi -relationships, each entity to one of the connection table. Example

Consider the following scene:

In this model,

is used as a custom connection table with additional attributes

and

.

public class Member
{
    public int MemberID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public virtual ICollection<MemberComment> MemberComments { get; set; }
}

public class Comment
{
    public int CommentID { get; set; }
    public string Message { get; set; }
    public virtual ICollection<MemberComment> MemberComments { get; set; }
}

public class MemberComment
{
    [Key, Column(Order = 0)]
    public int MemberID { get; set; }
    [Key, Column(Order = 1)]
    public int CommentID { get; set; }

    public virtual Member Member { get; set; }
    public virtual Comment Comment { get; set; }

    public int Something { get; set; }
    public string SomethingElse { get; set; }
}

Inquiry relationship MemberComment Something SomethingElse To query these relationships, you can use Linq expression:

The advantages of this method

This alternative method has the following advantages:

// 查找 LastName = "Smith" 的成员的所有评论
var commentsOfMembers = context.Members
    .Where(m => m.LastName == "Smith")
    .SelectMany(m => m.MemberComments.Select(mc => mc.Comment))
    .ToList();

// 查找 LastName = "Smith" 的所有成员及其评论
var membersWithComments = context.Members
    .Where(m => m.LastName == "Smith")
    .Select(m => new
    {
        Member = m,
        Comments = m.MemberComments.Select(mc => mc.Comment)
    })
    .ToList();

Custom connection table: You can control the design of the connection table and add other attributes as needed.

Inquiry:

You can screen and connect according to the attributes in the connection table.

Delay loading:
• You can use the delay loading function of EF to access related entities by connecting tables.
Summary
• Although EF does not directly support the multiple -to -multiple relationships with a custom connection table, the alternative method described here provides a flexible way to handle such relationships in the data model. This method provides the advantages of customizing, querying, and delay loading, so that you can meet your specific needs.

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