How to Implement Many-to-Many Relationships with Extra Fields in EF Core Code First?

The multiple -to -multiple relationships with additional fields in EF Core Code FIRST "

Introduction:

Entity Framework Core (EF Core) allows creating multiple relationships between entities. However, EF Core does not directly support the connection table of customizing these relationships. This article discusses a replacement method that uses Code First to create multiple -relationships with additional fields in the association table.

Create correlation:

We will create two pairs of multi -relationships instead of direct and more relationships. For example, consider the following scene:

In this scenario, the MEMBER and Comment entities have multiple relationships, and MemberComment represents the associated table of additional fields.

Query:

<code class="language-csharp">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 int Something { get; set; }
    public string SomethingElse { get; set; }
    public virtual Member Member { get; set; }
    public virtual Comment Comment { get; set; }
}</code>

To query the association with additional fields, you can use technology called "projection":

Operation:

In this model, adding entities and relationships similar to other CODE FIRST models:

<code class="language-csharp">// 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();</code>

Conclusion:

Although EF Core does not directly support the multiple -to -multiple relationships in the associated tables in the association table, it can be used to create similar functions with two pairs of multi -relationships. This method allows the creation of rich correlation with additional information to achieve more flexible data modeling and query functions.

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