Card-one relationships


In Molecule we model a cardinality-one relationship in our schema definition file with the one[<RefNamespace>] syntax:

object YourDomainDefinition {
  trait Person {
    val name = oneString
    val home = one[Addr]
  trait Addr {
    val street = oneString
    val city   = oneString

The ref attribute home has a card-one relationship to namespace Addr. When our schema is then translated to Molecule boilerplate code our home ref attribute is accessible as a value: === List(("Fred", 102))

This can be practical when we want to get a related entity id like 102 in this case.

Ref namespace

More often though we want to collect the values of the referenced entity attributes. Molecule therefore also creates an Uppercase method Home that allow us to jump to the Home (Addr) namespace and collect values of the entity 102 attributes: === ("Fred", "Baker St. 7", "Boston")

Describing the relationship we can simply say that a “Person has an Address”. It’s important to understand though that the namespace names Person and Addr are not like SQL tables. So there is no “instance of a Person” but rather “an entity with some Person attribute values” (and maybe other attribute values). It’s the entity id that tie groups of facts/attribute values together.

One-to-one or one-to-many

If Fred is living by himself on Baker St. 7 we could talk about a one-to-one relationship between him and his address.

But if several people live on the same address each person entity would reference the same address entity and we would then see each persons relation to the address as a one-to-many relationship since other persons also share the address: === List(
  ("Fred", 102),
  ("Lisa", 102),
  ("Mona", 102)

Wether a relationship is a one-to-one or one-to-many relationship is therefore determined by the data and not the schema.

Relationship graph

Relationships can nest arbitrarily deep. We could for instance in the Addr namespace have a relationship to a Country namespace and then get the country name too: === ("Fred", "Baker St. 7", "Boston", "USA")

And so on…


Card-many relationships…