How does the mapping functionality perform joins under hood ?

Hello Team,

I have a modeling question.

We want to add some well information to all object types in our knowledge graph. At the same time, we don't want to create a link from the objects to the well.

So, to avoid data duplication and updating these pieces of information, I thought about using the mapping functionality.

For example, I have a casing object with two attributes: name and diameter. And I added a name_well attribute using the mapping functionality, which, as I understand, under the hood, performs a join.

type Well {
  name: String
}

type Casing {
  name: String
  name_well: String @mapping(container: "Well", property: "name")
  diameter: Float32
}

Well transformation:

select
  wh.IDWELL as externalId,
  wh.WELLNAME as name
from
  well-data as wh

Casing transformation

select
  cas.IDCASING AS externalId,
  cas.NAME AS name,
  cas.DIAMETER as diameter,
  wh.WELLNAME as name_well
from
  casing-data as cas
  join well-data as wh on cas.IDWELL == cas.IDWELL

What I don’t understand is how exactly this join is performed under the hood.