= Mapping foreign-keys


The relational model:

```
orders (
    id (PK),
    customer_fk,
    ...
)

customers (
    id (PK),
    ...
)

FK "customer_orders" : orders.cust_fk -> customers.id
```

So we have a foreign-key between `orders` and `customers` where:

key-side:: `orders.customer_fk`
target-side:: `customers.id`


Assuming bi-directionality, we have 2 `Association` refs:

* `Order#customer` which models the key-side of the FK,
* `Customer#orders` which models the target-side



There is a single `ForeignKeyDescriptor` instance for this FK in our metamodel, with 2 Sides:

```
ForeignKeyDescriptor (
    name : "customer_orders",
    keySide (
        nature: KEY,
        column(s) : `orders.customer_fk`,
        keyModelPart : BasicType(...),
        ...
    ),
    targetSide (
        nature: TARGET,
        column(s) : `customer.id`,
        keyModelPart : BasicType(...),
        ...
    )
)
```

So this gives us all the information we need.  We just need to model that within the `Association` contract:

```
ManyToOne (
    name: `Order#customer`,
    foreignKeyDescriptor : FKD(customer_orders),
    side : FKD(customer_orders)#keySide
)

OneToMany (
    name: `Customer#orders`,
    foreignKeyDescriptor : FKD(customer_orders),
    side : FKD(customer_orders)#targetSide
)
```


When rendering one of these associations into SQL, we have all of the information we need.  Let's
assume we are processing HQL like `from Order join fetch customer`.  We know:

1. We be "coming from" the `Order` side, meaning the LHS will be `Order` and the RHS will be `Order#customer`
2. `Order#customer` knows that it models the Side(KEY) of the FKD(customer_orders).

Further, `Order#customer` also knows that the "other side" is, well, the other Side (here, the TARGET) modeled on the FKD.

All of this allows us to properly render join predicate.


If we come from the other side: `from Customer join fetch orders`, similar situation:

1. Here we are "coming from" the Customer side, meaning the LHS will be `Customer` and RHS is `Customer#orders`.
2. `Customer#orders` knows it is the `Side(TARGET)` of the FKD(customer_orders).

