Represent several relations between same types

[nicoulaj]

Example problematic cases:

1. Entities A et B have several different many-to-many relations
2. Entity A has several one-to-many relations to entity B (Eg: COUPLE.member1 -> USER and COUPLE.member2 -> USER)

In these cases there are several issues:

1. the codegen uses A/B entity names to generate Relation variants, so we obtain duplicate variants. I guess it could use the join table or foreign key entity name
2. the Related trait is typed on A/B entities, so same issue. So I believe fixing this requires a breaking conception change...

[tyt2y3]

Related was not designed to support this scenario and I want to keep its current definition for its simplicity of usage.

Goods news is, we had thought about how to define advanced relations with more than two level joins, and it would be another triat concept. Perhaps something like:

trait Linked {
    type FromEntity: EntityTrait;
    type ToEntity: EntityTrait;
    fn link(&self) -> Vec<RelationDef>;
}

such that multiple Link could be defined between any pair of Entities.

[tyt2y3]

Some clarifications:

Related is for inherit relations, like order with order item and product with product attribute, basically an edge on the ER diagram.

While Linked is the connection between any two Entity on the ERD, which can jump through multiple hops, and have multiple valid routes.

[tyt2y3]

We target to introduce new APIs in 0.2.0
Meanwhile you can use the 'unbounded' join method (like in https://www.sea-ql.org/SeaORM/docs/advanced-query/more-join)