少林一对一关系

Question

I am using Shaolinq library for database usage and I have problem with one-to-one relation. I have following database schema:

create table A (
    Id INT NOT NULL,
    PRIMARY KEY (`Id`)
);
create  table B (
    Id INT NOT NULL,
    PRIMARY KEY (`Id`),
    CONSTRAINT `fk_a_b_id` FOREIGN KEY (`Id`) REFERENCES `A` (`Id`)
);

basically those two tables contains info about same things, just different data. That's how someone has designed it, I cannot change it.

However, in the code I would like to have all those data in one object. I have been trying with BackReference, but no luck:

[DataAccessObject]
public abstract class A : DataAccessObject<uint>
{
    [AutoIncrement]
    [PersistedMember]
    public abstract uint Id { get; set; }

    [RelatedDataAccessObjects]
    public abstract RelatedDataAccessObjects<B> Bs { get; }
}

[DataAccessObject]
public abstract class B : DataAccessObject
{
    [BackReference(Name = "", PrefixName = "", SuffixName = "")]
    public abstract A Id { get; set; }

    [PersistedMember]
    public abstract string MoreData { get; set; }
}

This created somehow similar schema (obviously Id in B is not primary key now), but after I fill data in DB, Bs property in A is not filled with references. Simply puting abstract property of type B in A also doesn't seem to work since same column is foreign key and primary key at once (Id).

Any advice is welcome.

Answer 1

Do you need to access a collection of Bs from As? It seems to me that B is the "super class" that encompasses data from both A and B . If that's the case, then you can just make A the primary key of B .

[DataAccessObject]
public abstract class A : DataAccessObject<uint>
{
    [PersistedMember(Name = "Id")]
    public abstract override uint Id { get; set; }

    [PersistedMember]
    public abstract string Data { get; set; }
}

[DataAccessObject]
public abstract class B : DataAccessObject<A>
{
    [PersistedMember(Name = "Id", PrefixName = "")]
    public abstract override A Id { get; set; }

    [PersistedMember]
    public abstract string MoreData { get; set; }
}

Note that I've overridden the Id properties to get the column names matching the SQL. The defaults have meant column names: AId and BId

Because the primary key is an object it's known in Shaolinq terminology as a "complex primary key". You can use the deflated references feature to retrieve objects just be their primary key without a roundtrip to the database (more here: https://github.com/tumtumtum/Shaolinq/wiki/Deflated-References )

You can query for a specific value of B:

// Need to define primary key using an anonymous object because it's not an uint but rather an A that contains an uint.
model.Bs.GetByPrimaryKey(new { Id = 100 } );

You can also query for a specific value of B using a deflated reference:

model.Bs.GetByPrimaryKey(model.As.GetReference(100));

You can also use references within normal LINQ queries:

model.Bs.Where(c => c.Id == model.As.GetReference(100) || c.Id == model.As.GetReference(101)).ToList();

You can directly access nested/complex primary keys within the query too:

model.Bs.Where(c => c.Id.Id == 100 || c.Id.Id == 101).ToList();

Note that the above three queries don't need to perform an implicit join be cause the complex primary key is flattened out and stored on the table B .

When you get back B , you can access B.Id.Id without a round-trip query but if you access B.Id.Data then that will cause B.Id to automatically inflate with a hidden round-trip to the database. To avoid this, you can use Include to automatically inflate B.Id with the first query.

model.Bs.Include(c => c.Id).Single(c => c.Id == model.As.GetReference(100));

Or more succinctly:

model.Bs.Include(c => c.Id).Single(c => c.Id.Id == 100);