EntityFramework: Model 1:0..1 relationship with fluent api + conventions - entity-framework

Question

I have the following classes generated from an edmx model:
public partial class A
{
public int Id { get; set; }
public virtual B B { get; set; }
}
public partial class B
{
public int Id { get; set; }
public virtual A A { get; set; }
}
The existing db doesn't use the EF default which expects A.Id to be the primary key of table B:
CREATE TABLE [dbo].[B] (
[Id] INT IDENTITY (1, 1) NOT NULL,
PRIMARY KEY CLUSTERED ([Id] ASC)
);
CREATE TABLE [dbo].[A] (
[Id] INT IDENTITY (1, 1) NOT NULL,
[BId] INT NULL,
CONSTRAINT [fk] FOREIGN KEY ([BId]) REFERENCES [dbo].[B] ([Id])
);
With an edmx model, I can explicitly configure the multiplicity of each end, but I haven't found how to get the equivalent model using the fluent-api. When I do something like the following and generate a new db, the foreign key gets placed in table A instead of table B.
modelBuilder.Entity<A>().HasOptional(a => a.B).WithRequired(b => b.A);
I'm guessing I need to use a convention, but so far I've been unable to get the desired output.
UPDATE:
The closest solution I've found so far is to use the following which generates the correct SQL in the db:
modelBuilder.Entity<A>()
.HasOptional(a => a.B)
.WithOptionalDependent(b => b.A)
.Map(c => c.MapKey("BId"));
However, it's conceptually modeled as a 0..1:0..1 relationship and I haven't found how to set a CASCADE delete rule that deletes B when A is deleted.

Answer 1

I wasn't able to find a direct solution, but using the following code seems to meet my requirements of preserving the existing schema and creating a conceptual model that has the same multiplicities & delete behaviors as my original edmx model.
I'd still be interested in any solutions that don't require updating the conceptual model during the post-processing IStoreModelConvention.
{
var overridesConvention = new OverrideAssociationsConvention();
modelBuilder.Conventions.Add(overridesConvention);
modelBuilder.Conventions.Add(new OverrideMultiplictyConvention(overridesConvention));
}
private class OverrideAssociationsConvention : IConceptualModelConvention<AssociationType>
{
...
public List<AssociationEndMember> MultiplicityOverrides { get; } = new List<AssociationEndMember>();
public void Apply(AssociationType item, DbModel model)
{
if (multiplicityOverrides.Contains(item.Name))
{
// Defer actually updating the multiplicity until the store model is generated
// so that foreign keys are placed in the desired tables.
MultiplicityOverrides.Add(item.AssociationEndMembers.Last());
}
if (cascadeOverrides.Contains(item.Name))
{
item.AssociationEndMembers.Last().DeleteBehavior = OperationAction.Cascade;
}
}
}
private class OverrideMultiplictyConvention : IStoreModelConvention<EdmModel>
{
private readonly OverrideAssociationsConvention overrides;
public OverrideMultiplictyConvention(OverrideAssociationsConvention overrides)
{
this.overrides = overrides;
}
public void Apply(EdmModel item, DbModel model)
{
overrides.MultiplicityOverrides.ForEach(o => o.RelationshipMultiplicity = RelationshipMultiplicity.One);
}
}