foreign key of association not included in columns for wildcard select from subquery

[patricebender]

Given the following query:

> q = cds.ql`SELECT from (SELECT from (select from ${Books} as inner { author } ) as mid ) as outer where author = 42`
cds.ql {
  SELECT: {
    from: {
      SELECT: {
        from: {
          SELECT: {
            from: { ref: [ 'sap.capire.bookshop.Books' ], as: 'inner' },
            columns: [ { ref: [ 'author' ] } ]
          },
          as: 'mid'
        }
      },
      as: 'outer'
    },
    where: [ { ref: [ 'author' ] }, '=', { val: 42 } ]
  }
}

the forSQL transformation misses bubbling up the foreign key from the innermost selects association author:

> q.forSQL()
cds.ql {
  SELECT: {
    from: {
      SELECT: {
        from: {
          SELECT: {
            from: { ref: [ 'sap.capire.bookshop.Books' ], as: 'inner' },
            columns: [ { ref: [ 'inner', 'author_ID' ] } ]
          },
          as: 'mid'
        },
        columns: []
      },
      as: 'outer'
    },
    where: [ { ref: [ 'outer', 'author_ID' ] }, '=', { val: 42 } ],
    columns: []
  }
}

→ This is due to a mechanism which prevents us from duplicating a foreign key for a wildcard select. This case kicks in if we have a flat CSN, which contains an association and it's foreign key. However, in this case, it does only hold the association, hence we should flatten the association.