EF Core relationship without foreign key

Question

How to setup a navigation property without declaring a foreign key?

I have two tables ( segment and category ), they can join using 2 columns (origin/destination) but they don't meet the foreign key constraint because they can have value that doesn't exist in the other table (so its not about null or not null)

Segment

Id   Date       OriginId  DestinationId
---------------------------------------
1    2020-01-10    1        2
2    2020-01-18    2        1
2    2020-02-05    1        3
4    2020-04-11    3        3

Category

Id  OriginId   DestinationId  Category
-----------------------------------------
1      1            2         Primary
2      2            1         Secondary
2      2            3         Primary

I want to know every Segment category. But not every segment exists in Category so some segments won't have a category.

This SQL works:

SELECT 
    s.*, c.name
FROM 
    Segment s
LEFT OUTER JOIN 
    Category c ON c.originId = s.originId AND c.destinationId = s.destinationId

This LINQ also works:

from s in Segment
join c in Category on new { s.OriginId, s.DestinationId } equals new { c.OriginId, c.DestinationId } into grouping
from c in grouping.DefaultIfEmpty()
select new { s, c };

But to use navigation, closest I got is this: I added a property on each class respectively and setup the relationship in context using fluent api

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Segment>()
        .HasOne(segment => segment.Category)
        .WithMany(category => category.Segments)
        .HasForeignKey(segment => new { segment.OriginId, segment.DestinationId })
        .HasPrincipalKey(category => new { category.OriginId, category.DestinationId })
        .IsRequired(false);
}

// then I can do
var query = Segments.Include(o => o.Category).ToList();

This works to retrieve records that already exists in the database.

But to insert a new record to Segment where originId and destinationId do not exists in Category it complains about originId and destinationId not meeting the foreign key constraint. And it's ok because they don't exist in the other table.

They actually aren't foreign keys, just a column to use for joining them, but I don't khow to set this using a navigation property + Fluent Api.

Answer 1

First:

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Usage", "EF1001:Internal EF Core API usage.", Justification = "<挂起>")]
    public class MigrationsModelDifferWithoutForeignKey : MigrationsModelDiffer
    {
        public MigrationsModelDifferWithoutForeignKey
            ([NotNull] IRelationalTypeMappingSource typeMappingSource,
            [NotNull] IMigrationsAnnotationProvider migrationsAnnotations,
            [NotNull] IChangeDetector changeDetector,
            [NotNull] IUpdateAdapterFactory updateAdapterFactory,
            [NotNull] CommandBatchPreparerDependencies commandBatchPreparerDependencies)
            : base(typeMappingSource, migrationsAnnotations, changeDetector, updateAdapterFactory, commandBatchPreparerDependencies)
        {
        }

        public override IReadOnlyList<MigrationOperation> GetDifferences(IModel source, IModel target)
        {
            var operations = base.GetDifferences(source, target)
                .Where(op => !(op is AddForeignKeyOperation))
                .Where(op => !(op is DropForeignKeyOperation))
                .ToList();

            foreach (var operation in operations.OfType<CreateTableOperation>())
                operation.ForeignKeys?.Clear();

            return operations;
        }
    }

How to use:

services.AddDbContext<MyDbContext>(options =>
{
    options.UseSqlServer(Default);
    options.ReplaceService<IMigrationsModelDiffer, MigrationsModelDifferWithoutForeignKey>();
});

Answer 2

Sorry - a bit late to this party.

I just went through a similar excercise. You could try this:

modelBuilder.Entity<Category>()
    .HasMany(category => category.Segments)
    .WithOne(segment => segment.Category)
    .HasPrincipalKey(category => new { category.OriginId, category.DestinationId })
    .IsRequired(false)
    .OnDelete(DeleteBehavior.NoAction);