EntityFramework Many-To-Many relationship

Currently EntityFramework doas not support Many-To-Many relationship. In order to achieve such us relation joint entity is needed. That creates uncoforable sytuation in which we have to constantly deal with aditional 'middleman' entity that usually serves no other porpues then satisfy Entity Framework. This project aims to show how to use JointCollectionFacade.cs that will manage joints entities automatically. So dealing with many-to-many relationship is as easy as using ICollection.

Example uses MySQL provider for EntityFrameworkCore named Pomelo with lazy loading proxies. To run it, just change database credentials.

Lazy-loading

• Please notice that in order to populate Facade, joint entity with it's properties has to be included. In example lazy-load proxies loading it automatically. Without proxies inclusion have to be done manually otherwise property Roles will be empty.

DbSet<User> Users = DatabaseContext.Accounts;
var usersWithRoles = Users.Include(u => u.UserRoleJoints).ThenInclude(j => j.Role).ToList();

• Without lazy-loading virtual reference on Joint entity properties is not needed. Its virtual to allow proxies to do their magic.

OData implementation

In order to correctly $expand collection facade i recommend to use DTO's with AutoMapper which will help with loading facade without exposing joint entity in EDM model.

Here is how to create EdmModel for DTO:

var builder = new ODataConventionModelBuilder();
var userType= builder.AddEntityType(UserDTO);
userType.Name = nameof(User); // I like to hide the fact it is DTO, at the end every entity in EDM should be DTO so adding sufix into every entry is pointless.
builder.AddEntitySet("Users", userType);

Here is example of AutoMapper config:

new MapperConfiguration(cfg =>
{
    cfg.CreateMap<User, UserDto>()
        .ForMember(dto => dto.Roles,
            opt => opt.MapFrom(a => a.UserRoleJoints.Select(j => j.Role).AsQueryable()));
});

And finally the controller:

[EnableQuery]
public IEnumerable<UserDto> Get()
{
    DbSet<User> users = Database.Users;
    return Mapper.ProjectTo<UserDto>(users);
}