Add model validation for nested dependents

[AndriySvyryd]

If there are no non-PK required properties.

See GraphUpdatesSqlServerTest.Owned.Required_one_to_one_are_cascade_deleted

Model:

Root.RequiredSingle is an optional dependent

1. Add r.Navigation(e => e.Single).IsRequired(); to mark Root.RequiredSingle.Single as required

2. Remove RequiredSingle1.Bool

[AndriySvyryd]

Related: #21488

[smitpatel]

I updated model according to above. Test still fails in assert after loading required graph.

When we load root, root->RequiredSingle materializes always because there is no differentiating property. root->RequiredSingle->Single is not materialized because the values are null in the bool column inside RequiredSingle2 class. Test seems like making wrong assertion. @AndriySvyryd ?

[smitpatel]

Investigating more - The model falls into a kind of invalid model.
RequiredSingle is optional but nested Single is required.
Due to lack of properties in RequiredSingle it acts as required hence we materialize the instance but we cannot materialize Single since it was deleted along with RequiredSingle (due to it being required).

We need to warn for this and also make the model also act like RequiredSingle is actually required.

[ajcvickers]

Based on current design, we should add model validation to detect when there is an optional dependent that doesn't have any required properties that are not shared.

[smitpatel]

Design meeting N notes:

• We will add a IModelFinalizingConvention which would log and convert the dependent to required dependent so that metadata can be read that way at runtime.
• We will also remove the code from SQL which tries to add rows for nested dependents.

[smitpatel]

Design meeting N+ M notes:

• We will add validation in model.
• If optional dependent doesn't have a required identifying property,
  • If there are no nested dependents then warn
  • If there are nested dependents then throw.

[smitpatel]

Older snapshots which are already having this kind of faulty model would start throwing exception (tests are failing in source rn).

What direction are we taking on it? @dotnet/efteam

[AndriySvyryd]

We don't run model validation on snapshots

[smitpatel]

Looks like test in model snapshot processor is failing because the runtime model is faulty. I will debug more.