Solving correlated subquery error in BigQuery

Recently I had to create an array whilst selecting from another array in Google BigQuery:

SELECT ARRAY(
  SELECT AS STRUCT
    b.Column
  FROM UNNEST(a.Parts) p
  LEFT OUTER JOIN TableB b ON b.JoinColumn = p.JoinColumn
)
FROM TableA a

Unfortunately trying to use an outer join prompts the error message: Error: Correlated subqueries that reference other tables are not
supported unless they can be de-correlated, such as by transforming
them into an efficient JOIN.'

The solution for me was to extract the outer join into a separate subquery and do a left join on that one instead:

WITH separateSubquery AS (
  SELECT *
  FROM TableA a,
    UNNEST(a.Parts) p
  LEFT OUTER JOIN TableB b ON b.JoinColumn = p.JoinColumn
)
SELECT ARRAY(
  SELECT AS STRUCT
    b.Column
  FROM UNNEST(a.Parts) p
  LEFT JOIN separateSubquery b ON b.Id = p.Id
)
FROM TableA a