Entity Framework 1to1 relationship via association table not working in EDMX

Question

I have the following tables in SQL Server database

Which has a 1-1 association table (FooBar) which has unique indexes on corresponding FooId, BarId, and the primary key is (FooId, BarId).

To be clear FooBar does not allow any FooId (due to unique constraint) to be in the table more than once neither can any BarId (due to unique constraint) be in the table more than once. This is what makes it a 1-1 associative table.

I want to have this association table instead of 1-1 relationship between Foo and Bar because in my real world scenario, Bar will have other relationships to different unrelated tables and I will want similar association tables (as opposed to adding new FK columns to Bar for each new table)

Which I then bring these tables into my EDMX designer. The relationship is brought in as a Many to Many instead of One to One.

Which of course isn't what I want. I can manually change the model to a 1-1 relationship.

But then I get an error (in the designer).

Is this a bug or is it not possible to create a 1-1 association in this manner in EF?

Answer 1

It is a "bug" with the entire EF design: Entity Framework 4-6.1x Only Honors Multiplicity on Primary Keys.

Thus even though we know (and the RA models) that it is a 1-1 relationship due to a Candidate Key Constraint, EF does not and will not like it. Tough luck.

The "solutions" include:

1. Changing the model to something EF understands (EF understands Code First, not RA). Granted this may indicate an "issue" with the selected RA model, but such is orthogonal to the question ..

2. Live with the incorrectly generated multiplicity rules and "use with care"; the dirty work can be wrapped, but has to be added manually outside of the auto-generated model.

3. .. Hmm, others?

Shameless plug to unresolved questions deal with the same core lack-of feature:

• How to get EF6 to honor Unique Constraint (on FK) in Association/Relationship multiplicity?

• How to add an EF6 Association to a Candidate Key / Unique Key which is not the Primary Key?

Answer 2

The relationship you've shown in your first graphic is not a 1to1 relationship as far as EF is concerned.

It's a many to many relationship between Foo and Bar

Think about it this way:

Possible combinations with the following Foo and Bar values

Foo
1
2
3

Bar
1
2
3

FooBar
1, 1
1, 2
1, 3
2, 1
2, 2
2, 3
3, 1
3, 2
3, 3

Your FooBar table is a composite key, meaning it's a combination of the Foo and Bar values making up the key - not a 1 to 1 relationship

to define a 1 to 1 relationship between Foo and Bar, your schema should look something more like this:

Foo
FooId PK

Bar
FooId PK FK to Foo.FooId

the FooBar table is not needed for a 1to1 relationship between foo and bar.

As you stated in your question/comments - yes you put a unique constraint on the individual parts of your composite key, but EF doesn't take into account unique constraints for your model when determining a relationship. If you want a 1to1 relationship, you should create a 1to1 model, rather than mocking a 1to1 relationship via unique constraints.