bug in sql compiler

This seems like a bug because it's producing invalid SQL, but it's also possible I'm doing something wrong.

I have the following mapping, where views are joined by a compound key. Note lines marked as 1 and 2.

ObjectMapping(
  tpe = ConstraintSetGroupType,
  fieldMappings = List(
    SqlField("key", ConstraintSetGroupView.ConstraintSetKey, key = true, hidden = true),
    SqlField("program_id", ConstraintSetGroupView.ProgramId, key = true, hidden = true),
    SqlObject("observations",
      /* 1 */ Join(ConstraintSetGroupView.ProgramId, ObservationView.ProgramId),
      /* 2 */ Join(ConstraintSetGroupView.ConstraintSetKey, ObservationView.ConstraintSetKey),
    ),
  )
)

object ConstraintSetGroupView extends TableDef("v_constraint_set_group") {
  val ProgramId = col("c_program_id",  program_id)
  val ConstraintSetKey = col("c_conditions_key", text)
  ...
}

object ObservationView extends TableDef("v_observation") {
  val ProgramId = col("c_program_id", program_id)
  val ConstraintSetKey = col("c_conditions_key", text)
  ...
}

GraphQL Schema and query look like this:

type Query {

  # Observations grouped by commonly held constraints
  constraintSetGroups(
    programId: ProgramId!
    ...
  ): [ConstraintSetGroup!]! 

}

# example query
query {
  constraintSetGroups(programId: "p-101") {
    observations {
      id
    }
  }
}

The elaborator discards all the arguments for now, so Select(constraintSetGroups, Nil, child) is what the compiler is seeing.

If I comment out 1 above, the query is correctly compiled to:

SELECT
  v_observation.c_observation_id,
  v_constraint_set_group.c_conditions_key,
  v_constraint_set_group.c_program_id
FROM
  v_constraint_set_group
  LEFT JOIN v_observation ON (
    v_observation.c_conditions_key = v_constraint_set_group.c_conditions_key
  )

If I comment out 2 above, the query is correctly compiled to:

SELECT
  v_observation.c_observation_id,
  v_constraint_set_group.c_conditions_key,
  v_constraint_set_group.c_program_id
FROM
  v_constraint_set_group
  LEFT JOIN v_observation ON (
    v_observation.c_program_id = v_constraint_set_group.c_program_id
  )

However if I leave them both in, it's compiled to this, which is invalid SQL because table
v_constraint_set_group is introduced twice. It's also quite complicated.

SELECT
  v_observation_v_constraint_set_group_nested.c_observation_id,
  v_constraint_set_group.c_conditions_key,
  v_constraint_set_group.c_program_id
FROM
  v_constraint_set_group
  LEFT JOIN v_constraint_set_group ON (
    v_constraint_set_group.c_program_id = v_constraint_set_group.c_program_id
  )
  LEFT JOIN LATERAL (
    SELECT
      v_observation.c_conditions_key AS c_conditions_key_alias_0,
      v_observation.c_observation_id
    FROM
      v_observation
      INNER JOIN v_constraint_set_group ON (
        v_constraint_set_group.c_conditions_key = v_observation.c_conditions_key
      )
  ) AS v_observation_v_constraint_set_group_nested ON (
    v_observation_v_constraint_set_group_nested.c_conditions_key_alias_0 = v_constraint_set_group.c_conditions_key
  )

I would have expected something like:

SELECT
  v_observation.c_observation_id,
  v_constraint_set_group.c_conditions_key,
  v_constraint_set_group.c_program_id
FROM
  v_constraint_set_group
  LEFT JOIN v_observation ON (
    v_observation.c_conditions_key = v_constraint_set_group.c_conditions_key,
    v_observation.c_program_id = v_constraint_set_group.c_program_id
  )

Start by tracing the compiler path from the elaborator's Select(constraintSetGroups, Nil, child) through ObjectMapping and the two Join entries. Compare generated SQL for each join separately and together, checking how table aliases and nested lateral joins are introduced. Done means the compound-key query produces valid SQL without introducing v_constraint_set_group twice, while the single-join cases remain correct.