EF Core get Navigation Properties of an entity from Model with multiplicity Zero or One

Question

I am creating a generic class to seed the database with Entities, needed for integration tests. I can create individual entities but when one depends from another, I have to manually instruct my class first create the parent entity and then proceed. I am trying to make this detection automatic, getting from the Model definition, the list of navigation properties with multiplicity 0 or 1 (Reference navigation properties), once that is done, recursively my class will invoke itself to create parent entities first (Circular dependencies are out of scope here).

I used to do this in EF for .net Framework, but EF Core has changed a lot. What I am missing in EF Core is the RelationshipMultiplicity, I cannot find any reference to Multiplicity in the official documentation, and even tough the hacky solution is to check if the navigation property is a collection, I would like to have more control and keeps things simple.

So far I am exploring the Model definition using:

var modelData = _context.Model.GetEntityTypes()
    .Select(t => new
    {
        t.ClrType.Name,
        DerivedNavigationProperties = t.FindDerivedNavigations(t.ClrType.Name),
        DefiningNavigationProperties = t.FindDefiningNavigation(),
        DeclaredForeignKeys = t.GetDeclaredForeignKeys(),
        DeclaredNavigations = t.GetDeclaredNavigations(),
        DerivedNavigations = t.GetDerivedNavigations(),
        DerivedNavigationsInclusive = t.GetDerivedNavigationsInclusive(),
        Navigations = t.GetNavigations() // This returns all Navigation Properties (INavigation)
    });

Answer 1

After inspecting the source code in GitHub, I can say with enough confidence that there is no such thing as Multiplicity in EF Core.

I created an enumeration like the one used .net Framework 3.5+ (See: Official documentation):

public enum RelationshipMultiplicity
{
    Many = 2,
    One = 1,
    ZeroOrOne = 0
}

And then an extension method which allows getting all navigation properties using the enum as a filter. The key things I used are:

• Extension method for INavigation called IsCollection
• ForeignKey.IsRequired property

The method allows to get all Navigations properties by relation type

public static class ModelExtensions
{
    /// <summary>
    /// Extension method used to get from the entity all navigation properties by multiplicity
    /// </summary>
    /// <typeparam name="T">Entity from where the navigation properties are taken</typeparam>
    /// <param name="model">Context Model</param>
    /// <param name="multiplicity">Type of multiplicity to use</param>
    /// <returns>List of PropertyInfo of Navigation Properties</returns>
    public static IEnumerable<PropertyInfo> GetNavigationProperties<T>(this IModel model, RelationshipMultiplicity multiplicity)
    {
        var navigations = model.GetEntityTypes().FirstOrDefault(m => m.ClrType == typeof(T))?.GetNavigations();
        var properties = new List<PropertyInfo>();

        switch (multiplicity)
        {
            case RelationshipMultiplicity.Many | RelationshipMultiplicity.ZeroOrOne:
                return navigations?
                    .Select(nav => nav.PropertyInfo);
            case RelationshipMultiplicity.Many:
                return navigations?
                    .Where(nav => nav.IsCollection())
                    .Select(nav => nav.PropertyInfo);
            case RelationshipMultiplicity.One:
                return navigations?
                    .Where(nav => !nav.IsCollection() && nav.ForeignKey.IsRequired)
                    .Select(nav => nav.PropertyInfo);
            case RelationshipMultiplicity.ZeroOrOne:
                return navigations?
                    .Where(nav => !nav.IsCollection())
                    .Select(nav => nav.PropertyInfo);
            default:
                return null;
        }

        return properties;
    }
}

Usage example:

var oneToManyRelations = _context.Model.GetNavigationProperties<Transaction>(
    RelationshipMultiplicity.ZeroOrOne);

var manyToOneRelations = _context.Model.GetNavigationProperties<Transaction>(
    RelationshipMultiplicity.Many);

var allRelations = _context.Model.GetNavigationProperties<Transaction>(
    RelationshipMultiplicity.Many | 
    RelationshipMultiplicity.ZeroOrOne);