Add ForeignKeyConstraint to the relational model.

[AndriySvyryd]

Part of #12846

[smitpatel]

Can you also update your images from presentation to demonstrate where do ForeignKeyConstraint fit in those charts.

[bricelam]

Isn't this wrong? The store should restrict.

[AndriySvyryd]

🤷‍
@ajcvickers Thoughts?

[ajcvickers]

Did this PR change this? (I don't remember the mappings off the top of my head.)

[AndriySvyryd]

@ajcvickers No, this is what we have right now

[ajcvickers]

Then it's probably what we decided. Whether or not it is "correct" is a different question.

[AndriySvyryd]

TLDR: It doesn't matter

It's a bit confusing (as expected): for EF ReferentialAction.NoAction means use the database default and in most databases NoAction is the default. In most databases NoAction doesn't mean that no error is thrown, it means that the error is delayed until the transaction is committed. Also in SQL Server there's no Restrict.

EF currently doesn't support deleting and inserting the same (principal) row in a single transaction, so NoAction and Restrict are equivalent, but if we ever do then NoAction would be better.

[bricelam]

Fine to leave as is for now, but I want to do a deep review of these. I remember Diego and I really wanted to break something here.

[AndriySvyryd]

Can you also update your images from presentation to demonstrate where do ForeignKeyConstraint fit in those charts.

I don't think it's worth it, it's mostly IForeignKey.GetConstraintMappings() <*-*> IForeignKeyConstraint.MappedForeignKeys. The other aspects are straightforward.