fix(expand): proper subquery from construction (#1126)

if a query selects from another query, we need to consider the case
where the subquery has path expressions (and therefore joins).

The best solution is then to drill down into the innermost query of the
from clause, because we do not know the depth.

It is also not a good idea to check for the `ref` of the innermost
_untransformed_ query, as it is potentially a scoped query (as shown in
one of the tests).

Moreover, it is not possible to use the `ref` of the _transformed_ query
because it might be replaced by join `args`.

replaces #1114

fix #1112

Author: patricebender

db-service/lib/cqn4sql.js

@@ -801,8 +801,9 @@ function cqn4sql(originalQuery, model) {
         })
     } else {
       outerAlias = transformedQuery.SELECT.from.as
+      const getInnermostTarget = q => q._target ? getInnermostTarget(q._target) : q
       subqueryFromRef = [
-        ...(transformedQuery.SELECT.from.ref || /* subq in from */ transformedQuery.SELECT.from.SELECT.from.ref),
+        ...(transformedQuery.SELECT.from.ref || /* subq in from */ [getInnermostTarget(transformedQuery).name]),
         ...ref,
       ]
     }

db-service/test/cqn4sql/expand.test.js

@@ -1092,6 +1092,93 @@ describe('Unfold expands on associations to special subselects', () => {
     }`
     expect(JSON.parse(JSON.stringify(cqn4sql(q, model)))).to.eql(expected)
   })
+
+  it('expand via subquery with path expressions', () => {
+    const q = cds.ql`SELECT from (SELECT from bookshop.Books as inner { author, ID } where author.name = 'King') as Outer {
+      ID,
+      author { name }
+    }`
+    const res = cqn4sql(q, model)
+    const expected = cds.ql`
+      SELECT from (
+        SELECT from bookshop.Books as inner
+          left join bookshop.Authors as author on author.ID = inner.author_ID {
+            inner.author_ID,
+            inner.ID
+          } where author.name = 'King'
+        ) as Outer
+      {
+        Outer.ID,
+        (
+          SELECT from bookshop.Authors as $a {
+            $a.name
+          }
+          where Outer.author_ID = $a.ID
+        ) as author
+      }`
+    expect(JSON.parse(JSON.stringify(res))).to.deep.equal(expected)
+  })
+  it('expand via subquery with path expressions nested', () => {
+    const q = cds.ql`SELECT from (SELECT from (SELECT from bookshop.Books as inner { author, ID } where author.name = 'King') as Mid { * }) as Outer {
+      ID,
+      author { name }
+    }`
+    const res = cqn4sql(q, model)
+    const expected = cds.ql`
+      SELECT from (
+         SELECT from (
+          SELECT from bookshop.Books as inner
+          left join bookshop.Authors as author on author.ID = inner.author_ID {
+            inner.author_ID,
+            inner.ID
+          } where author.name = 'King'
+         ) as Mid {
+          Mid.author_ID,
+          Mid.ID 
+         }
+        ) as Outer
+      {
+        Outer.ID,
+        (
+          SELECT from bookshop.Authors as $a {
+            $a.name
+          }
+          where Outer.author_ID = $a.ID
+        ) as author
+      }`
+    expect(JSON.parse(JSON.stringify(res))).to.deep.equal(expected)
+  })
+  it('expand via subquery with path expressions and scoped query', () => {
+    const q = cds.ql`SELECT from (SELECT from bookshop.Books:genre as inner { parent, ID } where parent.name = 'Drama') as Outer {
+      ID,
+      parent { name }
+    }`
+    const res = cqn4sql(q, model)
+    const expected = cds.ql`
+      SELECT from (
+        SELECT from bookshop.Genres as inner
+          left join bookshop.Genres as parent on parent.ID = inner.parent_ID {
+            inner.parent_ID,
+            inner.ID
+          } where
+          exists (
+            SELECT 1 from bookshop.Books as $B
+            where $B.genre_ID = inner.ID
+          )
+          and parent.name = 'Drama'
+        ) as Outer
+      {
+        Outer.ID,
+        (
+          SELECT from bookshop.Genres as $p {
+            $p.name
+          }
+          where Outer.parent_ID = $p.ID
+        ) as parent
+      }
+    `
+    expect(JSON.parse(JSON.stringify(res))).to.deep.equal(expected)
+  })
 })
 
 describe('Expands with aggregations are special', () => {