EF Cosmos Embedded Inheritance

In this repository I will attempt to demonstrate an issue I have run into with the EF Core Cosmos DB provider.

This exploration started with me exploring my options for having a navigation property which corresponds to a TS union type.

Let's image we have a TS union type like this:

type Lookup =
  | { rule: 'by-first-name'; firstName: string; }
  | { rule: 'by-last-name'; lastName: string; }
  | { rule: 'by-employment-in-date-and-time-range'; from: Date; to: Date; }
  ;

This type allows one to store a rule which gets restored and executed on demand to produce search results. We want to store these queries in a structured way because we only allow some queries not a fully general query language.

For simplicity let's say each admin can have a single favorite filter stored with their profile:

type Admin = {
  firstName: string;
  lastName: string;
  favoriteLookup: Lookup;
};

When using EF Core at the BE with the Cosmos provider, to store this, we might model our entities like this:

public abstract class Lookup {
  public Guid Id { get; set; }
}

public sealed class LookupByFirstName {
  public string FirstName { get; set; }
}

public sealed class LookupByLastName {
  public string LastName { get; set; }
}

public sealed class LookupByEmploymentInDateAndTimeRange {
  public DateTime From { get; set; }
  public DateTime To { get; set; }
}

public sealed class Admin = {
  public string FirstName { get; set; }
  public string LastName { get; set; }
  public Lookup FavoriteLookup { get; set; }
};

This by itself should work. We will need to add HasBaseType most likely, but then it should work.

Check out the first demo showing this.

Note that OmniSharp is getting confused with multiple projects in different folders without SLN files so all demos have a corresponding SLN file.

This will not work without a base type - so with the base type of object. EF Core explicitly doesn't support navigation properties with object or dynamic types.

You can see the second demo where this is attempted and throws.

By default EF Core will never embed anything unless configured by OwnsOne or OwnsMany.

• Demonstrate OwnsOne and OwnsMany with a non-inheriting entity

However, HasBaseType is only available to the Entity top-level method in the model builder. It is not available further in the fluent API chain, so we will not be able to express that a top-level entity (Admin) has a navigation property (FavoriteLookup) whose values is a union of possible types with a shared base (Lookup).

This is either a bug or a missing feature or a technical limitation of EF Core. I want to find out which is it, so I am creating this demo to attach to a GitHub issue I will created in EF Core.

Or so I thought… But the third demo clearly shows it is possible to have an embedded entity which is at the same time a union type.

This contrasts with my other experiment, ef-cosmos-union-type, where turning on OwnsOne causes a model validation exception I also face in the real app I am building.

So there must be something between this simple schema in demo3 and the more complex one in ef-cosmos-union-type which causes the error and it might be the longer embeddance chain.

To-Do

Embed Admin into something else to prolong the embedance chain to see if that causes an error

Try to use OwnsMany and introduce multiple favorites