support multiple entity references of same type [DATAJDBC-125]

[spring-projects-issues]

Jens Schauder opened DATAJDBC-125 and commented

I did not test this, but if I understand the code correctly which I wrote for DATAJDBC-112 if an entity contains multiple references to entities of identical type, the repository will break, because the property name is not part of the column name used for the FK.

Example

A Customer hast two properties of type Address (home and work.

The EntityRowMapper will try to load both properties by joining from Customer.id to Address.customer or similar. Instead something should be used like Address.home_customer.

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Referenced from: pull request #91

[spring-projects-issues]

Jens Schauder commented

The solution for this is actually fairly easy but the question is: does it make sense. The problem is: there are many ways one might implement such references. Assume we have an entity A with two properties x and y both referencing a B. Here is how this might be look in the database

1. Two references from A

A(id, x_b, y_b); B(id)

2. Two references from B. This is what we would get with the feature, which really looks silly.

A(id); B(id, x_a, x_b)

3. Switching column

A(id); B(id, a, role)

role being a String or something determining which property this B is used for.

4. Join tables

A(id); B(id); ABx(a, b); ABy(a,b)

5. Single join tables

A(id); B(id); AB(a, b, role);

The one we can easily support right now is the one I have never seen in an actual system.

Therefore I'm postponing this until we at least can support having the FK on the referencing side. See DATAJDBC-128.

[spring-projects-issues]

Jens Schauder commented

This should now be doable with the help of DATAJDBC-218. 

 

With an added test this could be closed