Add support for @JoinColumn

Hi,

I love the simplicity of spring-data-jdbc, but sometimes it feels too restrictive compared to possibilities of JPA. I miss the @JoinColumn feature.

Please let me describe my need for it

class Order {
   @Id
   String id;                        // technically, randomly generated value without _any_ business meaning, e.g. UUID
   String reference ;           // some kind of business information encoded in the value and it is _unique_ (!)
   @MappedCollection
   Set<OrderItem> orderItems;
   // might be a better name compared to @JoinColumn
   //@MappedColumn
   @JoinColumn("reference")  // <== not using the "id" field
   private Meta meta;
}

class OrderItem {
   @Id
   Long id;                      // technical id
   String orderId;            // back reference to Order
}

class Meta {
   @Id
   String reference;
   String data;
}

I have use cases, where relations are either based on that unique field/property/column or on another unique field/property/column.
I tried to use the NamingStrategy to achieve my goal, but it didn't work either.

best regards

Start by reviewing the existing NamingStrategy path and the Order/Meta mapping example in the issue. Trace how relational associations currently choose their referenced column, then identify the tests and mapping entry points that would need coverage for a unique non-id column. Done means spring-data-jdbc supports the requested @JoinColumn-style association without relying on NamingStrategy.