如何在实体框架中使用linq返回此结果（2个外键映射表）

Question

I have a table that contains 2 foreign key that reference separately to 2 different table. I would like to return the result of all person that has course of "Science".

How to retrieve the record back using LINQ? This is what i gotten so far:

       return 
        _ctx.Person
            .Include(u => u.Course
            .Where(ug=>ug.CourseName== "Science"));

This is not working as it shows the error.

The Include path expression must refer to a navigation property defined on the type

public class Course
{
    public int CourseID {get; set;}
    public string CourseName {get; set;}

    public virtual ICollection<Person> Persons { get; set; }
}

public class Person
{
   public int PersonID {get; set;}
   public virtual ICollection<Course> Courses { get; set; }
}

This is the mapping table. Only contains 2 foreign key from 2 different table. I could not use this table inside the solution.As the code first won't generate this table as it doesn't contain it's own PK.

//This is not shown in the EntityFramework when generating Code First.
    public class PersonCouseMap 
    {
      public int PersonID {get; set;}
      public int CourseID {get; set;}
    }

Update : this works after I switched the entity.

return _ctx.Course
       .Include(u=>u.Person)
       .Where(ug=>ug.CourseName == "Sciene");

Anyone can explain why it won't work the another way round. I need to display a List of Person who have course of "Science", not Course Science that has a list of user.

Answer 1

The original query does not work because you've pushed the Where predicate inside the Include expression, which is not supported as indicated by the exception message.

The Include method is EF specific extension method used to eager load related data. It has nothing to do with the query filtering.

To apply the desired filter person that has course of "Science" you need Any based predicate since the Person.Courses is a collection:

return _ctx.Person
    .Where(p => p.Courses.Any(c => c.CourseName == "Science"));

To include the related data in the result, combine it with Include call(s):

return _ctx.Person
    .Include(p => p.Courses)
    .Where(p => p.Courses.Any(c => c.CourseName == "Science"));

Answer 2

It looks like there is no relations between these two entites, you can establish a relationship by making the following changes to your code:

Here I am assuming that you want to establish Many-to-Many relationship between these two tables by having a third entity PersonCourseMap

public class Course
{
    public int CourseID {get; set;}
    public string CourseName {get; set;}

   public virtual ICollection<CoursePersons> Courses { get; set; }
}

public class Person
{
   public int PersonID {get; set;}
   public virtual ICollection<PersonCourse> Courses { get; set; }
}

public class PersonCourseMap 
{
  public int PersonID {get; set;}
  public int CourseID {get; set;}

  public virtual ICollection<Person> Persons { get; set; }
  public virtual ICollection<Course> Courses { get; set; }
}

After making above changes you can simply navigate through properties.

Answer 3

Include Foreign Key Mapping

public class Course
{
    public int CourseID {get; set;}
    public string CourseName {get; set;}
    public virtual ICollection<Person> Person {get; set}
}

public class Person
{
   public int PersonID {get; set;}
   public virtual ICollection<Course> Course {get; set;}
}

using System.ComponentModel.DataAnnotation.Schema;
public class PersonCouseMap 
{
   [ForeignKey("Person")]
   public int PersonID {get; set;}
   [ForeignKey("Course")]
   public int CourseID {get; set;}

   public virtual ICollection<Person> Person {get; set;}
   public virtual ICollection<Course> Course {get; set;}
}