Azure CosmosDb + EF Core: Can't use embedded collections inside owned properties?

We're using Cosmos db with EF Core.

We have a model like this:

class Root 
{
   public Child? Child { get; set; }
}

class Child 
{
   public List<GrandChild> GrandChildren { get; set; }
}

Configured as follows:

modelBuilder.Entity<Root>().OwnsOne(x => x.Child);

However, I can't figure out how to configure the GrandChildren array within the owned property Child.

Without any configuration, I get this error:

> System.InvalidOperationException : Unable to determine the relationship represented by navigation 'Child.GrandChildren' of type 'List<GrandChild>'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

After attempting to add configuration:

modelBuilder.Entity<Child>().HasMany(x => x.GrandChildren);

Now I face a different error:

> System.NullReferenceException : Object reference not set to an instance of an object.
>
> Stack Trace: 
> InternalEntityTypeBuilder.CreateForeignKey(InternalEntityTypeBuilder
> principalEntityTypeBuilder, IReadOnlyList1 dependentProperties, Key
> principalKey, String propertyBaseName, Nullable1 required,
> ConfigurationSource configurationSource)
> InternalEntityTypeBuilder.HasRelationship(EntityType targetEntityType,
> Nullable1 navigationToTarget, Nullable1 inverseNavigation,
> Nullable1 setTargetAsPrincipal, ConfigurationSource
> configurationSource, Nullable1 required)
> InternalEntityTypeBuilder.HasOwnership(TypeIdentity& targetEntityType,
> MemberIdentity& navigation, Nullable1 inverse, ConfigurationSource
> configurationSource)
> InternalEntityTypeBuilder.HasOwnership(TypeIdentity& typeIdentity,
> MemberIdentity navigation, ConfigurationSource configurationSource)
> OwnedNavigationBuilder2.OwnsManyBuilder[TNewRelatedEntity](TypeIdentity
> ownedType, MemberIdentity navigation)
> OwnedNavigationBuilder2.OwnsMany[TNewDependentEntity](Expression1
> navigationExpression)
> <>c.<OnModelCreating>b__13_4(OwnedNavigationBuilder2 cb) line 39
> EntityTypeBuilder1.OwnsOne[TRelatedEntity](Expression1
> navigationExpression, Action1 buildAction)
> ConfigurationDbContext.OnModelCreating(ModelBuilder
> modelBuilder) line 32 ModelCustomizer.Customize(ModelBuilder
> modelBuilder, DbContext context)

Apparently this is a limitation of EF Core provider for cosmosdb that owned items cannot have owned properties of their own?

>I can't figure out how to configure the GrandChildren array within the owned property Child.

I tried with the below code which stores GrandChildren array within the owned property Child.

public class YourDbContext : DbContext
{
    public DbSet<Root> Roots {get; set;}

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        var endpoint = "*****";
        var key = "******";
        var databaseName = "nestedDb";

        optionsBuilder.UseCosmos(endpoint, key, databaseName);
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Root>().OwnsOne(x => x.Child, cb =>
        {
            cb.OwnsMany(c => c.GrandChildren);
        });
    }
}

class Program
{
    static void Main(string[] args)
    {
        using (var dbContext = new YourDbContext())
        {
            dbContext.Database.EnsureCreated();

            var grandChild1 = new GrandChild { Id = 1, Name = "Pavan", Age = 25 };
            var grandChild2 = new GrandChild { Id = 2, Name = "Balaji", Age = 20 };

            var child = new Child
            {
                GrandChildren = new List<GrandChild> { grandChild1, grandChild2 }
            };

            var root = new Root
            {
                Child = child
            };

            dbContext.Roots.Add(root);
            dbContext.SaveChanges();

            Console.WriteLine("Root and related entities saved to Cosmos DB.");
        }
    }
}

Output:

Explanation:

• Initially define the Entities such as Root, Child and GrandChild.

• YourDbContext class derives from DbContext, which Entity Framework Core provides for interacting with databases.

• The Roots property is a DbSet that represents the database's collection of Root entities.

• OnConfiguring and OnModelCreating configures connection details and relationship between Root, Child, and GrandChild respectively.

• The two GrandChild entities are added to the collection of GrandChildren of a Child entity.

• The Child entity is allocated to the Child property of the Root object.

• The data is saved to the Cosmos DB database by using the dbContext.SaveChanges() function.

• The OwnsOne and OwnsMany methods in the OnModelCreating method assist in defining the ownership connections between entities. One Child belongs to The Root, and that Child belongs to numerous GrandChild entities.

• For further please refer this link.