Entity Framework Core + MariaDB - Navigation Property is null - mariadb

Question

I have a many-to-many relationship defined using a table in MariaDB. I am trying to use Navigation Properties in Entity Framework Core for the related entities, but they're not hydrated.
roles
| role_id | role_name |
| 1 | Role-A |
| 2 | Role-B |
groups
| group_id | group_name |
| 1 | Group-A |
| 2 | Group-B |
role_to_group
| role_to_group_id | role_id | group_id |
| 1 | 1 | 1 |
| 2 | 2 | 1 |
| 3 | 2 | 2 |
This is the class
[Table(name: "role_to_group")]
public class RoleToGroup
{
[Column(name: "role_to_group_id")]
public long RoleToGroupId { get; set; }
[Column(name: "role_id")]
[ForeignKey("RGToRoles")]
public int RoleId { get; set; }
[Column(name: "group_id")]
public int UserGroupId { get; set; }
[Include]
public Role Role { get; set; }
[Include]
public UserGroup UserGroup { get; set; }
}
The [Include] is a custom defined attribute for hydrating the navigation properties using the pattern similar to DbContext.Set<T>().Include("property_name") via an extension method that does reflection on T and finds all properties with the attribute [Include] defined on them.
However, I am unable to get this to work correctly. The navigation properties Role and UserGroup return null.
What do you suggest I am doing wrong? Also please let me know if you need any more details to be added to the question!!

Answer 1

The problem might be that EF Core doesn't automatically do lazy loading. ( Documentation about how to implement lazy loading ).
When installed these NuGet packages:
Microsoft.EntityFrameworkCore.Proxies
Microsoft.EntityFrameworkCore.Relational
and enabled lazy loading as e.g.:
public class MyDbContext : DbContext
{
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseMySQL(#"User Id=root;Host=localhost;Database=Test;");
optionsBuilder
.UseLazyLoadingProxies();
}
public DbSet<UserGroup> UserGroups { get; set; }
public DbSet<Role> Roles { get; set; }
public DbSet<RoleToGroup> RoleToGroups { get; set; }
}
I got it loaded:
using (var context = new MyDbContext())
{
var roleGroup1 = context.RoleToGroups.First();
Console.WriteLine(roleGroup1.Role.Name); //Role-A
Console.WriteLine(roleGroup1.UserGroup.Name); //Group-A
}
Lazy loading requires types to be public, and properties to be lazy-loaded must be virtual:
[Table(name: "role_to_group")]
public class RoleToGroup
{
[Column(name: "role_to_group_id")]
public int RoleToGroupId { get; set; }
[Column(name: "role_id")]
[ForeignKey("roles")]
public int RoleId { get; set; }
[Column(name: "group_id")]
[ForeignKey("groups")]
public int UserGroupId { get; set; }
public virtual Role Role { get; set; } //must be virtual
public virtual UserGroup UserGroup { get; set; }//must be virtual
}
[Table(name: "roles")]
public class Role
{
[Key]
[Column(name: "role_id")]
public int RoleId { get; set; }
[Column(name: "role_name")]
public string Name { get; set; }
}
[Table(name: "groups")]
public class UserGroup
{
[Key]
[Column(name: "group_id")]
public int GroupId { get; set; }
[Column(name: "group_name")]
public string Name { get; set; }
}
Alternatively to lazy loading, manual Eager or Explicit loading could be used from the same doc.